High-strength steel sheets and processes for production of the same

ABSTRACT

A high strength steel sheet with both excellent elongation and stretch-flanging performance is provided. 
     The high strength steel sheet of the present invention comprises, in percent by mass, C: 0.05 to 0.3%, Si: 0.01 to 3.0%, Mn: 0.5 to 3.0%, Al: 0.01 to 0.1%, and Fe and inevitable impurities as the remainder, and has a structure mainly composed of tempered martensite and annealed bainite. The space factor of the tempered martensite is 50 to 95%, the space factor of the annealed bainite is 5 to 30%, and the mean grain size of the tempered martensite is 10 μm or smaller in terms of the equivalent of a circle diameter. The steel sheet has a tensile strength of 590 MPa or higher. 
     The high strength steel sheet of the present invention has a space factor of the martensite phase which is a main component of the metal structure is 80% or higher; the mean grain size of the martensite phase is 10 μm or smaller in terms of the equivalent of a circle diameter; in the martensite phase, the space factor of the martensite phase having a grain size of 10 μm or larger in terms of the equivalent of a circle diameter is 15% or lower; and the space factor of the retained austenite phase in the metal structure is 3% or lower. 
     The high strength steel sheet of the present invention is a dual phase steel sheet mainly composed of a ferrite phase and martensite, and the space factor of the ferrite phase is 5 to 30%, and the space factor of the martensite phase is 50 to 95%. Moreover, the ferrite phase is annealed martensite.

TECHNICAL FIELD

The present invention relates to a high strength steel sheet for which high press formability is required, typically including steel sheets for automobiles, particularly to a high strength steel sheet with both elongation and stretch-flanging performance and a method for manufacturing the same.

BACKGROUND ART

High strength steel sheets, which are generally used by being press-molded, are used in industrial product such as automobiles, electric devices and industrial machines. Since high strength steel sheets are used for the purpose of lightening industrial products, they need not only have high strength, but also have the ability to form various configurations of the products. Accordingly, it is required for high strength steel sheets to have excellent press formability. To meet this requirement, high-strength steel sheets having excellent elongation and stretch-flanging performance, which are necessary for improving press formability, are required.

Examples of known steels having such characteristics include dual phase steel (DP steel) whose metal structure is composed of a ferrite phase and a martensite phase, as described in Patent document 1. Since this DP steel can ensure ductility (elongation) due to its soft ferrite and strength due to its rigid martensite, it has both strength and elongation (in particular, uniform elongation). However, because of the coexistence of soft ferrite and rigid martensite, distortion (stress) is concentrated at the interface of the two phases when deformed, and therefore the interface is likely to serve as the starting point of rupture, thereby disadvantageously preventing ensuring stretch-flanging performance (local elongation).

Examples of steel sheets which expectedly have ductility (especially, uniform elongation) higher than those of DP steels include TRIP steels utilizing the TRIP (Transformation Induced Plasticity) phenomenon, as described in Patent document 2. This TRIP steel is a steel sheet in which uniform elongation is increased by transforming retained austenite into martensite during deformation (working-induced transformation). However, since martensite which has been transformed from retained austenite in the TRIP steel is extremely hard, it likely serves as the starting point of rupture, lowering the stretch-flanging performance of the steel sheet.

Other methods of improving the stretch-flanging performance of the high strength steel sheets include that in which the metal structure is single-phase structure and localization of process distortion is suppressed by homogenizing the in the metal structure, and that in which a difference in strength between a soft phase having a multi-phase metal structure and a hard phase is reduced.

Since martensite single-phase structure steel sheet has a uniform structure, it is known as a steel sheet which has both strength and stretch-flanging performance. However, the martensite single-phase structure steel sheet disadvantageously has low ductility, and insufficient elongation.

Patent document 3 discloses a high-stretch-strength cold-rolled steel sheet in which martensite single-phase structure is achieved by justifying the composition and heat treatment conditions of the steel sheet, and tensile strength is 880 to 1170 MPa. That is, the high-stretch-strength cold-rolled steel sheet of Patent document 3 is produced by heating and retaining a steel sheet having a predetermined composition range at 850° C., which is normally reachable temperature industrially, to transform the steel sheet into austenite, and then rendering it a martensite single-phase structure. A steel sheet of a martensite single-phase structure produced by this invention has a tensile strength of 880 to 1170 MPa, and thus has excellent stretch-flanging performance. However, it has elongation EL (%) lower than 8% and thus has low ductility. In the high strength steel sheet of the invention of Patent document 3, if ductility is improved, press formability can be further improved.

Moreover, Patent document 4 discloses a method for manufacturing a high tensile strength steel sheet, in which a steel sheet in which the ratio by volume of a low-temperature transformation phase comprising a martensite phase and others and a retained austenite phase is 90% or higher of the entire metal structure is heated and retained to produce a two phase region: a ferrite phase and an austenite phase, a metal structure comprising a fine ferrite phase which has succeeded the laths of the low-temperature transformation phase and the austenite phase is provided, and finally the steel sheet is given such a metal structure that comprises ferrite and the low-temperature transformation phase finely dispersed in the form of laths.

However, since the steel sheet produced by the steelmaking method disclosed in Patent document 4 has a relatively high cooling stop temperature in the steelmaking process, a large amount of bainite is deposited, while a large amount of retained austenite also remains therein, and therefore the steel sheet has excellent ductility, but has insufficient stretch-flanging performance. By the steelmaking method of Patent document 4, a steel sheet which is excellent in both elongation and stretch-flanging performance cannot be produced.

[Patent document 1] Japanese Unexamined Patent Application Publication (JP-A) No. S55-122820

[Patent document 2] JP-A-S60-43425

[Patent document 3] Japanese Patent No. 3729108

[Patent document 4] JP-A-2005-272954

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

As mentioned above, since DP steel sheets, TRIP steel sheets, and martensite single-phase structure steel sheets have their advantages and disadvantages, a steel sheet which has high strength and excellent elongation and stretch-flanging performance at the same time is required. The present invention has been made to solve such a problem, and an object thereof is to provide a high strength steel sheet excellent in both elongation and stretch-flanging performance and a method for manufacturing the same.

Another object of the present invention is to provide a high strength steel sheet having a tensile strength of 780 MPa or higher, in which elongation and stretch-flanging performance are both improved, and a method for manufacturing the same.

Means for Solving the Problem

The high strength steel sheet of the present invention is constituted of, in percent by mass, C: 0.05 to 0.3%, Si: 3% or less (not including 0%.), Mn: 0.5 to 3.0%; Al: 0.01 to 0.1%, and the remainder comprising iron and inevitable impurities, has a space factor of a martensite phase which is a main component of a metal structure of 50% or higher, and has a tensile strength of 590 MPa or higher.

To this end, the inventors of the present invention have studied various structures that can ensure high strength and improve elongation, especially stretch-flanging performance at the same time. As a result, the inventors found the following: by annealed bainite, which is a fine lath-shaped structure, as an initial structure in a two phase temperature region of ferrite+austenite (hereinafter referred to as “two-phase region annealing”.), fine annealed bainite produced in a base material acts in a manner of suppressing the growth of austenite, fine tempered martensite is produced from austenite by the following hardening and tempering, and the entire structure is formed from these microstructures. Therefore, elongation and stretch-flanging performance are improved. The inventors accomplished the present invention based on these findings.

That is, the high strength steel sheet of the present invention has a structure mainly comprising tempered martensite and finely dispersed annealed bainite, a space factor of the tempered martensite of 50 to 95%, a space factor of the annealed bainite of 5 to 30%, and a mean grain size of the tempered martensite in terms of the equivalent of a circle diameter of 10 μm or lower. The term “equivalent of a circle diameter” means the diameter of an anticipated circle having the same area as the grains of tempered martensite, and is determined by subjecting a structure picture to image analysis. Moreover, the term “space factor” means the percentage by volume, and is determined by corroding a structure observation test piece with nital, observing the test piece with an optical microscope (1000 times), and by subjecting the observed structure picture to image analysis. Moreover, annealed bainite is observed as a body centered cubic structure in terms of a crystal structure.

The method for manufacturing a high strength steel sheet with excellent elongation and stretch-flanging performance according to the present invention comprises using a steel sheet having a space factor of bainite in the entire metal structure of 90% or higher as a material steel sheet; heating and retaining the steel sheet at a temperature of (Ac₃ point −100° C.) or higher but not higher than Ac₃ point for 0 to 2400 seconds (including 0 seconds), and then cooling to a transformation start temperature of martensite, Ms point, or lower at an average cooling rate of 10° C./sec. or higher, subsequently heating and retaining the steel sheet at a temperature of 300 to 550° C. for 60 to 1200 seconds. The high strength steel sheet of the present invention is thus produced. The material steel sheet can be produced by hot rolling a steel piece having the above-mentioned chemical component or further by cold rolling the same.

Herein, Ac₃ point is a temperature at which a two-phase region comprising an austenite phase and a ferrite phase transforms into an austenite single-phase region that is stable at high temperatures in a temperature raising step.

The inventors of the present invention have also invented a high strength steel sheet having a limited ratio by volume of the retained austenite phase of 3% or lower, which does not affect stretch-flanging performance, and a metal structure in which a large part of the metal structure is a fine martensite phase.

That is, in the high strength steel sheet of the present invention, the space factor of the martensite phase which is a main component of the metal structure is 80% or higher; the mean grain size of the martensite phase is 10 μm or smaller in terms of the equivalent of a circle diameter; the space factor of a martensite phase having a grain size of 10 μm or larger in terms of the equivalent of a circle diameter in the martensite phase is 15% or lower, and the space factor of the retained austenite phase in the metal structure is 3% or lower.

Herein, space factor means a ratio by volume of each phase constituting the metal structure in the steel material to the entire metal structure. the space factors of the martensite phase and ferrite phase were determined by subjecting the steel material to repeller corrosion, observing the material by an optical microscope and an SEM (1000 times), and then subjecting the material to image analysis. The space factor of the retained austenite phase was determined by the saturation magnetization method (refer to “Netsushori” (heat treatment), Vol. 136, (1996)). Moreover, the mean grain size of the martensite phase is the mean value of the crystal grain size of the martensite phase, and is determined by structure analysis using a FE/SEM-EBSP at step intervals of 100 nm in the present invention.

In the metal structure of the high strength steel sheet, the space factor of the fine tempered martensite phase having a mean grain size of 10 μm or smaller is 80% or higher, and therefore a tensile strength of 780 MPa or higher and excellent ductility are ensured. Moreover, when the space factor of the retained austenite phase is high, stretch-flanging performance is lowered. However, the space factor of the retained austenite phase is limited to 3% at the highest in the present invention, and therefore stretch-flanging performance is not lowered.

In the high strength steel sheet, it is preferable that the martensite phase is a tempered martensite phase; an annealed martensite phase is contained as the metal structure other than the martensite phase and the retained austenite phase; and that the space factor of the annealed martensite phase is 3 to 20%.

Such features suppress combination between the crystal grains of the austenite phase and the growth of the same by the finely dispersed annealed martensite phase. As a result, the final structure is micronized, and the processability of the high strength steel sheet is ensured.

The method for manufacturing the high strength steel sheet according to the present invention is for manufacturing a high strength steel sheet of the present invention by using a steel sheet in which the total space factor of the martensite phase and/or of the retained austenite phase in the entire metal structure is 90% or higher as a material steel sheet, heating and retaining the steel sheet at a temperature of (Ac₃ point-100° C.) or higher, Ac₃ point or lower for 30 to 1200 seconds, cooling the steel sheet to a transformation start temperature of martensite, Ms point, or lower at an average cooling rate of 10° C./sec. or higher, and further conducting a heat treatment in which the steel sheet is heated and retained at a temperature of 300 to 500° C. for 60 to 1200 seconds.

In the high strength steel sheet of the present invention, the structure which is the main part of the metal structure is a martensite phase and a ferrite phase; the space factor of the martensite phase is 50 to 95% (meaning “% by volume”, and so on); the space factor of the ferrite phase is 5 to 30%; and the mean grain size of the martensite phase is 10 μM or smaller in terms of the equivalent of a circle diameter.

The ferrite phase is preferably an annealed martensite.

The method for manufacturing the high strength steel sheet according to the present invention is for manufacturing a high strength steel sheet of the present invention by using, as a material steel sheet, a steel sheet in which the total space factor of the martensite phase and/or bainite phase in the entire metal structure is 90% or higher and the grain size of the former austenite is 20 μm or smaller in terms of the equivalent of a circle diameter, heating and retaining the steel sheet at a temperature of (Ac₃ point −100° C.) or higher but not higher than Ac₃ point for 1 to 2400 seconds, then cooling the steel sheet to a transformation start temperature of martensite, Ms point, or lower at an average cooling rate of 10° C./sec. or higher, and subsequently conducting a heat treatment in which the steel sheet is heated and retained at a temperature of 300 to 550° C. for 60 to 1200 seconds.

The high strength steel sheet according to the present invention may comprise, in addition to the above-mentioned basic components, any of the element groups (a) to (e) described below, or one or more elements selected from a plurality of groups within a range defined for each element group.

(a) an element selected from Ti, Nb, V and Zr: 0.01 to 1% by mass in total

(b) Ni and/or Cu: 1% by mass or less in total

(c) Cr: 2% by mass or less and/or Mo: 1% by mass or less

(d) 0.0001 to 0.005% by mass of B

(e) Ca and/or REM: 0.003% by mass or less in total

Effect of the Invention

In the present invention, a structure which is mainly composed of especially tempered martensite and finely dispersed annealed bainite is provided, wherein the space factors thereof are defined to have predetermined amounts, and the mean grain size of tempered martensite is defined 10 μm or smaller. Accordingly, a high strength steel sheet which has strength as high as 590 MPa or higher, excellent elongation and stretch-flanging performance, and thus excellent press formability can be provided.

According to the present invention, it is also possible to provide a high strength steel sheet in which the space factor of the retained austenite phase is 3% or lower and the space factor of the fine martensite phase is 80% or higher by a relatively simple heat treatment step. Since this high strength steel sheet has a tensile strength of 780 MPa or higher, and also has excellent elongation and stretch-flanging performance, it is excellent in press formability.

According to the present invention, it is also possible to achieve a high strength steel sheet which has excellent elongation and stretch-flanging performance at the same time by designing the steel sheet especially for a dual phase steel sheet mainly composed of a ferrite phase and martensite, ensuring high strength of the steel sheet as a whole, and appropriately controlling the space factors of especially the ferrite phase and martensite and the mean grain sizes of the same.

BEST MODE FOR CARRYING OUT THE INVENTION (1)

The best mode for carrying out the invention will be described below in detail.

A high strength steel sheet according to one embodiment of the present invention is designed to have a structure, as the main body, in which annealed bainite is finely dispersed in tempered martensite, a space factor of the tempered martensite of 50 to 95%, a space factor of the annealed bainite of 5 to 30%, a mean grain size of the tempered martensite of 10 μm or smaller in terms of the equivalent of a circle diameter, and a tensile strength of 590 MPa or higher. The reasons for limitation of the structure will be described below.

When the space factor of the annealed bainite is 5% or lower, the pinning effect, which suppresses the growth of austenite, is weak, and austenite grains grow so that martensite grains become large, thereby preventing ensuring good elongation. In contrast, when the space factor is higher than 30%, stretch-flanging performance is lowered. For this reason, the lower limit of annealed bainite is 5%, and preferably 7%, while its upper limit is 30%, and preferably 25%.

When the space factor of tempered martensite is lower than 50%, strength and stretch-flanging performance are lowered. In contrast, when the space factor is higher than 95%, the steel sheet becomes too hard and thus elongation is lowered. For this reason, the lower limit of the tempered martensite phase is 50%, and more preferably 70%, while its upper limit is 95%, and preferably 85%.

The mean grain size of the tempered martensite varies depending on the amount of annealed bainite finely dispersed. When the grain size is larger than 10 μm in terms of the equivalent of a circle diameter, elongation and stretch-flanging performance are lowered. For this reason, the upper limit is 10 μm.

The structure in which the tempered martensite and annealed bainite coexist constitutes the main part of the structure of the high strength steel sheet of the present invention. Herein, the main part means 90% or higher, and preferably 95% or higher, and other structures contained in an amount of less than about 10% are permitted because they hardly affect elongation, especially stretch-flanging performance. Examples of other structures include ferrite, pearlite, retained austenite and the like. Of course, the less these structures, the better.

Chemical component (unit: % by mass) which is preferable for obtaining the structure and strength of the steel sheet according to the present invention will be described now. Examples of such a chemical component include that comprises the followings: C: 0.05 to 0.3%, Si: 0.01 to 3.0%, Mn: 0.5 to 3.0%, and Al: 0.01 to 0.1%, and Fe and inevitable impurities as the remainder. The reasons for component limitation will be described below.

[C: 0.05 to 0.3%]

C is an important element in producing martensite, and increasing the strength of the steel sheet. When the amount of C is lower than 0.05%, such an effect is excessively lowered. In contrast, from the perspective of increasing strength, the higher the amount of C, the more preferable. However, when the amount of C is higher than 0.3%, a large amount of retained austenite is produced and stretch-flanging performance is lowered. Moreover, weldability is also deteriorated. For this reason, the lower limit of the amount of C is 0.05%, and preferably 0.07%, while its upper limit is 0.3%, and preferably 0.25%.

[Si: 0.01 to 3.0%]

Si acts as a deoxidizing element when steel is melted, and is an element effective in increasing strength without deteriorating the ductility of steel. Si also acts to suppress deposition of coarse carbide which deteriorates stretch-flanging performance. When the amount of Si is lower than 0.01%, these actions are excessively lowered, while addition of the same in an amount higher than about 3.0% saturates the effect. For this reason, the lower limit of the amount of Si is 0.01%, and preferably 0.1%, while its upper limit is 3.0%, and preferably 2.5%.

[Mn: 0.5 to 3%]

Mn is an element useful in increasing the hardening characteristics of steel to ensure high strength, but when its amount is lower than 0.5%, such an action is excessively lowered. In contrast, when its amount is higher than 3%, ductility is lowered and processability is thus adversely affected. For this reason, the lower limit of the amount of Mn is 0.5%, and preferably 0.7%, while its upper limit is 3%, and preferably 2.5%.

[Al: 0.01 to 0.1%]

Al is an element which has a deoxidation effect, and needs to be added in an amount of 0.01% or higher to perform the effect. In contrast, even if it is added in an amount higher than 0.1%, the deoxidation effect is saturated, and it becomes a source of non-metallic mediators to deteriorate physical properties and surface properties. For this reason, the lower limit of the amount of Al is 0.01%, and preferably 0.03%, while its upper limit is 0.1%, and preferably 0.08%.

Preferable chemical components of the steel sheet the present invention include, in addition to the above-mentioned basic components, Fe and impurities which inevitably get in, for example, P, S, N and O. However, to improve the mechanical characteristics of the steel sheet, any of the auxiliary element groups (a) to (e) described below, or one or more element selected from a plurality of groups may be added within the additional permissible range of each group.

(a) One or more elements selected from Ti, Nb, V and Zr in a total amount of 0.01 to 1%

(b) One or more elements selected from Ni and Cu in a total amount of 1% or lower

(c) One or more elements of Cr: 2% or lower, Mo: 1% or lower

(d) B in an amount of 0.0001 to 0.005%

(e) One or more elements selected from Ca and REM in a total amount of 0.003% or lower

[One or More Member of Ti, Nb, V and Zr: in a Total Amount of 0.01 to 1%]

These elements form precipitates such as carbides, nitrides, and carbonitrides together with C and N, and contribute to the improvement of strength. They also have an action to increase elongation and stretch-flanging performance by micronizing crystal grains during hot rolling. When the total amount of these elements added is 0.01%, such an action is excessively lowered. In contrast, when the amount is higher than 1%, elongation and stretch-flanging performance are lowered rather than increased. For this reason, the lower limit of the total amount of one or more of these elements is 0.01%, and preferably 0.03%, while its upper limit is 1.0%, and preferably 0.7%.

[One or More Members of Ni and Cu: in a Total Amount of 1% or Lower]

These elements are effective in maintaining the balance of strength and ductility high and realizing high strength at the same time. To effectively exhibit such an effect, it is preferable to add the elements in an amount of 0.05% or higher. Meanwhile, the higher the amount of these elements contained, the higher the above-mentioned effect, but when the total amount of one or more of these elements is higher than 1%, such an effect is saturated, and cracks may occur during hot rolling. For this reason, the upper limit of the total amount of these elements is 1.0%, and preferably 0.7%.

[One or More Members of Cr: 2% or Lower, Mo: 1% or Lower]

These elements are both effective in stabilizing the austenite phase, and facilitating the generation of bainite in the course of cooling. The higher the amount of the elements contained, the higher the effect, but when they are contained in an excessive amount, ductility is deteriorated rather than improved. For this reason, the amount of Cr is 2.0% or lower, and more preferably 1.5% or lower, while the amount of Mo is 1.0% or lower, and more preferably 0.7% or lower.

[B: 0.0001 to 0.005%]

B is an element effective in improving hardening characteristics, and increasing the strength of the steel sheet when added in a minute amount. To perform such an effect, it is preferable that the element is contained in an amount of 0.0001% or higher. However, when the amount of B contained is excessive and higher than 0.005%, crystal grain boundaries may be embrittled and cracks may occur during rolling. For this reason, the upper limit of the amount of B is 0.005%.

[One or More Members of Ca and REM: in a Total Amount of 0.003% or Lower]

These elements are effective in controlling the form of sulfide in the steel and improving processability. The higher the amount of the elements contained, the higher such an effect, but when they are contained in an excessively high amount, the above-mentioned effect is saturated. Therefore, the upper limit of the total amount of one or more members of these elements is 0.003%.

The method for manufacturing the high strength steel sheet according to an embodiment of the present invention will be now described. First, a material steel sheet which has the above-mentioned chemical components and a space factor of bainite to the entire structure of 90% or higher is prepared. Second, this material steel sheet is retained at a temperature of (Ac₃ point-100) C or higher but not higher than Ac₃ for 0 sec. or longer but nor longer than 2400 sec., and then an annealing heat treatment is carried out, in which the material steel sheet is cooled to the martensite transformation start temperature, Ms point, or lower at an average cooling rate of 10° C./sec. or higher. Subsequently, a tempering heat treatment is carried out, in which the material steel sheet is retained at 300° C. or higher but not higher than 550° C. for 60 sec. or longer but not longer than 1200 sec., whereby a microstructure steel sheet mainly composed of the tempered martensite and annealed bainite and having a tensile strength of 590 MPa or higher is obtained.

The material steel sheet can be produced by the steps described below. First, steel having the above-mentioned chemical components is melted, By using the steel slab, hot rolling is terminated in such a manner that the finishing temperature is not lower than Ar₃ point. Second, the steel slab is cooled at an average cooling rate of 10° C./sec. or higher to the bainite transformation temperature (about 350 to 450° C.), and is wound up at the same temperature. When the finishing temperature is lower than Ar₃ point or the cooling rate after the hot rolling is lower than 10° C./sec., a ferrite phase is likely to be produced in the hot-rolled steel sheet, and the space factor of bainite of the material steel sheet becomes lower than 90%. The material steel sheet used may be a cold-rolled steel sheet produced by hot rolling steelandthensubjectingthesteeltoanacidcleaningprocessandcoldrolling. In the steel types which contain Ti, Nb, V and Zr, to re-solutionize precipitates containing the elements produced before hot rolling, it is preferable to heat and retain the steel piece at a relatively high temperature during hot rolling.

As for the material steel sheet, the space factor of bainite can be made 90% or higher by subjecting a hot-rolled steel sheet which does not meet the above hot rolling condition and cooling condition to preliminary annealing. This preliminary annealing is a heat treatment in which a hot-rolled steel sheet is retained in a temperature range of Ac₃ point or higher for about 5 seconds, and then the steel sheet is cooled at an average cooling rate of 10° C./sec. or higher to the bainite transformation temperature. When the retaining temperature is lower than Ac₃ point, the ferrite phase is likely to be produced in the steel sheet, and the space factor of bainite is lowered. Even when the steel sheet is retained at a temperature Ac₃ point or higher, if the retaining time is shorter than about 5 seconds, transformation into austenite is insufficient, and therefore the space factor becomes lower than 90%. Even when the steel sheet is subjected to the preliminary annealing, it can be cold-rolled thereafter to prepare as a cold-rolled steel sheet, and this can be used as the material steel sheet.

After the material steel sheet is prepared, the material steel sheet is retained at a temperature (Ac₃ point −100)° C. or higher but not higher than Ac₃ for 0 sec. or longer (including 0 sec.) but not longer than 2400 sec., and then two-phase region annealing is carried out, in which the material steel sheet is cooled to the martensite transformation start temperature, Ms point, or lower at an average cooling rate of 10° C./sec. or higher, followed by tempering. By such a heat treatment, the structure of the high strength steel sheet according to the present invention is obtained. First, the conditions of the two-phase region annealing will be described below.

The reason why the annealing temperature of the two-phase region annealing is set to (Ac₃ point −100)° C. or higher but not higher than Ac₃ is as follows: When the annealing temperature is set to a temperature range higher than Ac₃ point in which the austenite single phase is stable, the crystal grains of austenite grow in the material steel sheet and combine with each other to become coarse, and the growth inhibitory effect (pinning effect) of austenite by finely dispersed annealed bainite cannot be obtained. For this reason, a fine dual phase steel sheet cannot be obtained, and the stretch-flanging performance of the high strength steel sheet is lowered. In contrast, if the steel sheet is annealed at a temperature lower than (Ac₃ point −100)° C., transformation into austenite does not proceed sufficiently, and the space factor of martensite after the heat treatment becomes lower than 50%, and the stretch-flanging performance of the steel sheet is thus lowered.

As for the annealing time (heating and retaining time), austenite having a space factor of about 50% and thus martensite can be obtained simply by heating the steel sheet to the annealing temperature, but the time is preferably 1 sec. or longer, and more preferably 5 seconds or longer. In contrast, if the material steel sheet is retained longer than necessary, austenite grains become coarse, and fine martensite cannot be obtained. Therefore, it is preferable that the retaining time is limited to 2400 sec. or shorter, and preferably 1200 sec. or shorter.

When the average cooling rate after heating and retaining is lower than 10° C./sec. or the cooling stop temperature is higher than the martensite transformation start temperature, Ms point, a retained austenite phase, a pearlite phase and a ferrite phase are produced, a cementite phase is deposited, and structures other than martensite are formed from austenite in large amounts, whereby elongation and stretch-flanging performance are lowered.

Tempering (reheating treatment) is carried out after the two-phase region annealing, which is a process for improving elongation and stretch-flanging performance by softening hard martensite, and decomposing retained austenite which produces martensite by working-induced transformation. Tempering conditions are as follows: the material steel sheet is retained at a temperature of 300° C. or higher but not higher than 550° C. for 60 sec. or longer but not longer than 1200 sec. The cooling rate after retaining is not especially limited.

When the tempering temperature is lower than 300° C., softening of martensite is insufficient, and the elongation and stretch-flanging performance of the steel sheet are lowered. In contrast, when the temperature is higher than 550° C., a coarse cementite phase is deposited, and the stretch-flanging performance of the steel sheet is lowered. For this reason, tempering is carried out at a temperature of 300° C. or higher but not higher than 550° C.

When the retaining time of tempering is shorter than 60 sec., softening of martensite is insufficient, while when the time is longer than 1200 sec., martensite is too softened, which makes ensuring strength difficult, and deposition of cementite lowers the stretch-flanging performance of the steel sheet. For this reason, the lower limit of the retaining time during tempering is 60 sec., preferably 90 sec. or longer, and more preferably 120 sec., and the upper limit is 1200 sec., preferably 900 sec., and more preferably 600 sec.

The present invention will be described in more detail below with reference to Examples, but the present invention should not be interpreted as being restricted by such Examples.

Example 1

Steel slabs having chemical compositions shown in Table 1 below were melted, and the steel slabs were heated to about 1000 to 1100° C. The steel slabs were subjected to hot rolling or further preliminary annealing under the conditions described in Table 2 below, producing material steel sheets. The average cooling rate after the hot rolling was 50° C./sec. test pieces for observing structures were collected from the material steel sheets, and the space factors of bainite were determined by observing structure constitutions with a microscope and subjecting microscope structure pictures after being corroded with nital to image analysis. The values of Ac₃ point and Ms point calculated from the components by a known equation are also shown in Table 1 for reference. Moreover, the results of structure observation are also shown in Table 2. The obtained material steel sheets were subjected to final annealing (two-phase region annealing) and tempering under the conditions shown in Table 3 below, producing sample steel sheets.

TABLE 1 Transformation temperature Steel Chemical components (% by mass) (° C.) symbol C Si Mn P S Al Others Ac₃ Ms Remarks A 0.11 1.21 1.62 0.011 0.001 0.044 — 873 448 Component of invention B 0.18 1.54 2.06 0.120 0.002 0.048 — 934 406 Component of invention C 0.01 0.88 1.56 0.012 0.002 0.044 — 908 487 Component of comparison D 0.08 1.86 2.29 0.016 0.001 0.039 — 894 433 Component of invention E 0.25 1.55 2.01 0.020 0.002 0.034 — 845 382 Component of invention F 0.35 1.51 2.01 0.012 0.002 0.033 — 819 346 Component of comparison G 0.18 0.05 2.05 0.009 0.001 0.031 — 783 406 Component of invention H 0.16 2.63 1.22 0.009 0.002 0.034 — 930 446 Component of invention I 0.21 3.52 1.99 0.011 0.001 0.038 — 938 398 Component of comparison J 0.14 1.54 0.38 0.009 0.003 0.039 — 913 486 Component of comparison K 0.13 1.56 0.62 0.009 0.001 0.038 — 909 480 Component of invention L 0.21 1.24 2.78 0.006 0.002 0.033 Zr: 0.021 806 367 Component of invention M 0.19 1.53 3.49 0.013 0.001 0.033 — 808 346 Component of comparison N 0.17 1.38 2.02 0.015 0.002 0.005 V: 0.018 842 409 Component of invention O 0.19 1.32 1.97 0.011 0.003 0.089 — 865 407 Component of invention P 0.17 1.42 2.06 0.012 0.001 0.167 — 903 413 Component of comparison Q 0.17 1.39 2.00 0.009 0.002 0.019 Ni: 0.2 842 411 Component of invention R 0.16 1.56 1.93 0.010 0.001 0.031 Cu: 0.1 860 418 Component of invention S 0.17 1.33 2.19 0.012 0.002 0.042 Cr: 0.35 841 397 Component of invention T 0.16 1.27 2.03 0.015 0.003 0.042 Mo: 0.1 855 414 Component of invention U 0.18 1.36 1.93 0.016 0.003 0.045 B: 0.0002 856 411 Component of invention V 0.17 1.40 1.97 0.014 0.002 0.039 Ca + REM: 0.001 855 413 Component of invention (Note) Remainder is Fe and inevitable impurities

TABLE 2 Hot-rolling Material steel conditions Preliminary annealing conditions sheet structure Finishing Cooling Space Sam- temper- Winding Heating Retaining Cooling stop factor ple Steel ature temperature temperature time rate temperature Phase of No. symbol ° C. ° C. ° C. sec. ° C./sec. ° C. constitution bainite % Remarks 1 A 930 550 900 120 50 400 B + γ 97 Conditions of invention 2 B 950 550 950 120 50 400 B + γ 94 Conditions of invention 3 C 870 500 930 90 20 420 B + α + γ 85 Conditions of comparison 4 D 860 550 930 240 50 400 B + γ 96 Conditions of invention 5 E 890 550 900 180 50 370 B + γ 92 Conditions of invention 6 F 850 500 930 120 50 350 B + γ 91 Conditions of comparison 7 G 800 500 910 60 40 400 B + γ 100 Conditions of invention 8 H 850 600 930 120 50 400 B + γ 91 Conditions of invention 9 I 900 550 930 120 50 400 B + γ 87 Conditions of comparison 10 J 900 500 930 60 50 400 B + γ 95 Conditions of comparison 11 K 900 550 930 120 50 430 B + γ 96 Conditions of invention 12 L 900 550 930 360 50 400 B + γ 94 Conditions of invention 13 M 850 500 900 120 50 400 B + γ 92 Conditions of comparison 14 N 900 500 860 120 50 400 B + γ 95 Conditions of invention 15 O 900 500 880 30 50 430 B + γ 94 Conditions of invention 16 P 900 550 930 180 50 400 B + γ 96 Conditions of comparison 17 Q 870 550 870 120 40 400 B + γ 93 Conditions of invention 18 R 880 550 890 120 50 400 B + γ 94 Conditions of invention 19 S 900 550 880 10 50 400 B + γ 94 Conditions of invention 20 T 900 550 870 120 50 430 B + γ 96 Conditions of invention 21 U 900 550 900 120 50 400 B + γ 93 Conditions of invention 22 V 900 550 890 120 50 400 B + γ 94 Conditions of invention 23 B 930 400 — — — — B + γ 92 Conditions of invention 24 B 930 420 — — — — B + γ 91 Conditions of invention 25 A 900 400 — — — — B + γ 96 Conditions of invention 26 B 930 350 — — — — B + γ 96 Conditions of invention 27 A 930 430 — — — — B + γ 98 Conditions of invention (Note) α: Ferrite, B: Bainite. γ: Austenite

TABLE 3 Tempering Final annealing conditions conditions Heating Retaining Cooling Cooling stop Heating Retaining Sample Steel temperature time rate temperature temperature time No. symbol ° C. sec. ° C./sec. ° C. ° C. sec. Remarks 1 A 850 180 500 20 400 180 Conditions of invention 2 B 850 180 500 20 400 120 Conditions of invention 3 C 850 200 100 20 500 180 Conditions of comparison 4 D 870 180 200 20 500 180 Conditions of invention 5 E 815 80 300 20 520 120 Conditions of invention 6 F 810 220 300 20 350 180 Conditions of comparison 7 G 750 120 300 100 400 120 Conditions of invention 8 H 910 350 300 50 500 180 Conditions of invention 9 I 870 100 200 20 350 120 Conditions of comparison 10 J 800 100 200 20 450 180 Conditions of comparison 11 K 850 180 500 20 520 180 Conditions of invention 12 L 770 120 300 20 500 180 Conditions of invention 13 M 770 180 200 20 400 180 Conditions of comparison 14 N 820 120 500 20 500 180 Conditions of invention 15 O 850 180 300 20 500 180 Conditions of invention 16 P 880 120 100 20 400 120 Conditions of comparison 17 Q 825 180 500 20 500 120 Conditions of invention 18 R 830 120 500 20 500 180 Conditions of invention 19 S 810 120 300 20 500 180 Conditions of invention 20 T 850 60 300 20 500 180 Conditions of invention 21 U 820 180 500 20 500 180 Conditions of invention 22 V 830 120 500 20 500 180 Conditions of invention 23 B 880 180 300 20 450 180 Conditions of invention 24 B 900 120 300 20 500 120 Conditions of invention 25 A 850 180 300 20 450 180 Conditions of invention 26 B 850 180 300 20 500 180 Conditions of invention 27 A 800 120 500 20 500 180 Conditions of invention

The structures (space factors of annealed bainite, space factors and mean grain sizes of tempered martensite), and mechanical characteristics (tensile strength TS, elongation EL and stretch-flanging performance) of the sample steel sheets were determined in the manner described below.

Test pieces for observing structures were collected from the sample steel sheets, and the space factors of annealed bainite and tempered martensite were determined by subjecting microscope structure pictures after being corroded with natal to image analysis. Moreover, the mean grain sizes of tempered martensite were determined by measuring the areas of the grains by structure analysis using FE/SEM-EBSP, determining the diameters of circles corresponding to the grains, and averaging the diameters.

Among the mechanical properties, tensile strength and elongation were determined by using a universal tensile tester manufactured by Instron and JIS No. 5 tensile test piece. Stretch-flanging performance was determined by measuring a hole expansion rate (λ) by using a 20-ton hole expansion tester manufactured by Tokyo Koki, according to The Japan Iron and Steel Federation standard (JFST 1001-1996), and was evaluated based on this. The results of these measurements are also shown in Table 4. In Table 4, as for “evaluation”, tensile strength (TS) of 590 MPa or higher, elongation (EL) of 10% or higher, and hole expansion rate (λ) of 80% or higher were rated excellent characteristics. The samples which were excellent in all three characteristics were rated 0; those which were excellent in two characteristics out of three were rated A; and those which were excellent in only one characteristic out of three were rated x.

TABLE 4 Structure parameters Mean Space Space grain size Mechanical factor factor of characteristics Sample Steel of of tempered M TS No. symbol annealed B % tempered M % μm MPa EL % λ % Evaluation Remarks 1 A 12 86 7.4 984 13.5 127.0 ◯ Example of invention 2 B 29 70 8.3 689 32.1 80.8 ◯ Example of invention 3 C 19 80 8.1 554 31.9 81.5 Δ Comparative example 4 D 12 86 7.3 992 11.9 114.2 ◯ Example of invention 5 E 13 84 7.8 1108 12.1 107.9 ◯ Example of invention 6 F 10 89 8.3 1388 6.7 53.2 X Comparative example 7 G 16 83 8.9 782 18.1 106.8 ◯ Example of invention 8 H 12 86 7.9 1022 12.9 104.0 ◯ Example of invention 9 I 25 76 9.1 1382 5.8 27.4 X Comparative example 10 J 35 65 8.8 588 28.8 64.9 X Comparative example 11 K 22 75 8.2 603 28.3 86.3 ◯ Example of invention 12 L 14 85 7.9 1109 12.5 100.5 ◯ Example of invention 13 M 12 85 8.1 1299 8.1 58.7 X Comparative example 14 N 11 88 7.3 1031 13.9 124.0 ◯ Example of invention 15 O 10 89 7.2 1017 14.7 127.9 ◯ Example of invention 16 P 13 86 8.1 1031 10.3 61.1 Δ Comparative example 17 Q 10 89 8.0 1022 14.3 122.8 ◯ Example of invention 18 R 13 86 7.9 1098 12.9 121.4 ◯ Example of invention 19 S 12 86 8.5 1139 10.9 114.9 ◯ Example of invention 20 T 7 92 8.8 1222 10.7 98.7 ◯ Example of invention 21 U 12 87 8.1 1154 11.1 104.8 ◯ Example of invention 22 V 12 86 7.9 1095 11.9 106.3 ◯ Example of invention 23 B 13 86 7.9 989 13.2 112.8 ◯ Example of invention 24 B 12 87 7.7 981 14.3 127.3 ◯ Example of invention 25 A 12 86 7.5 789 17.5 117.7 ◯ Example of invention 26 B 24 75 8.7 708 19.8 103.2 ◯ Example of invention 27 A 19 80 8.3 737 18.9 128.3 ◯ Example of invention (Note) B: Bainite, M: Martensite

It can be seen from Table 4 that the sample steel sheets (examples of the invention) sample Nos. 1, 2, 4, 5, 7, 8, 11, 12, 14, 15 and 17 to 27 in which the conditions of the present invention were met in terms of all of chemical components, material steel sheet structures, final annealing conditions and tempering conditions all have tensile strengths as high as 590 MPa or higher, elongations of 10% or higher, and stretch-flanging performances of hole expansion rates of 80% or higher. That is, it can be seen that these samples have high strength and yet excellent elongation and stretch-flanging performance, and excellent press formability.

(2)

Other embodiments of the present invention will be described below in detail.

First, the composition of constituents of the high strength steel sheet material of this embodiment will be described. The elements which constitute the composition of the high strength steel sheet of this embodiment are C, Si, Mn, Al, Cr, Mo, Nb, Ti and V, and the remainder is Fe and inevitable impurities. Among these constituent elements, Cr, Mo, Nb, Ti and V are not essentially necessary constituent elements, but are the elements which are added to further increase the effect of the present invention. The actions of the elements will be described below. In the description provided below, the proportions of compositional ranges are indicated by % by mass.

Among the constituent elements mentioned above, the compositional range of C is limited within the range of 0.05% to 0.3%. C is an element effective in producing the tempered martensite phase, and increasing the strength of the steel sheet material. The lower limit value, i.e., 0.05% is an amount which is minimally necessary to obtain desired strength. The upper limit value, i.e., 0.3%, is limited for the following reason. When C is added in an amount higher than the upper limit value, 0.3%, the concentrations of C in the tempered martensite phase and the retained austenite phase are increased, and the strength of these phases is increased. A difference in strength between these phases and the ferrite phase having a low concentration of C is increased. Since rupture is likely to occur at the interface of these phases having a difference in strength, stretch-flanging performance is lowered. Meanwhile, when the concentration of C in the steel sheet is increased, weldability is significantly deteriorated.

The compositional range of Si is limited within the range higher than 0% but not higher than 3%. Si has the action to inhibit the generation of relatively coarse carbide which lowers stretch-flanging performance, and also improve ductility. However, this action to improve ductility is saturated in an amount of Si added of about 3%. Moreover, since Si has the action to retard softening by tempering of the tempered martensite phase, when the amount of Si contained is high, the tempered martensite phase is not sufficiently tempered and thus strength is retained high, whereby a difference in strength between the martensite phase and the ferrite phase is increased and stretch-flanging performance is lowered. Accordingly, the upper limit of the amount of Si added is 3%.

The compositional range of Mn is limited within the range of 0.5% or higher but not higher than 3%. Mn has the effect to increase the tensile strength of the steel sheet by solid solubility reinforcement, improve the hardening characteristics of the steel sheet, and promote generation of the martensite phase. Such an action of Mn is found in steel having containing Mn in an amount of 0.5% or higher. Preferably, the amount of Mn contained is 1% or higher. In contrast, when the amount of Mn contained is higher than 3%, adverse effects such as cracks in the slab occur. The amount of Mn contained is preferably 2.5% or lower.

The compositional range of Al is limited within the range of 0.01% or higher not higher than 0.1%. Al is used for deoxidation of steel in the steelmaking process. When there is no solid solution of Al present in the metal structure of steel, deoxidation of steel may not be completed. When oxygen is remaining in steel, remaining oxygen is bonded to Si and Mn. Since these oxidation products of Si and Mn are likely to separate and float from the cast, the composition of steel becomes non-uniform and processability is lowered. Moreover, when the amount of Al solutionized in the metal structure of steel is higher than 0.1%, deoxidation products are reduced by Al again, and metal-like Al is produced. this metal-like Al serves as a relatively large mediator, and creates material defects or surface flaws. Therefore, the upper limit value of Al is 0.1%.

Cr and Mo are not elements essential to the high strength steel sheet of the embodiment, but their addition acts effectively. Cr and Mo act to inhibit the generation of carbide which lowers stretch-flanging performance, and promote the generation of the martensite phase in the metal structure of the steel sheet. Therefore, they can be added as needed. The compositional range of Cr and Mo is such that at least one or more elements selected from Cr and Mo is contained, and the total compositional ratio of these elements is 0.5% or lower. In order to effectively perform the action of Cr and Mo, it is recommended that the compositional proportions of Cr and Mo are 0.05% or higher (more preferably 0.1% or higher), respectively. However, even if Cr and Mo are added, whether singly or in combination of both, in an amount higher than 0.5%, the action mentioned above is saturated, and an action which is worth the amount of Cr and Mo contained cannot be obtained.

None of Nb, Ti and V is an element which is essential to the high strength steel sheet of this embodiment, but their addition acts effectively. Nb, Ti and V have the action to form carbonitride, increase the tensile strength of steel by enhancing deposition, and micronize crystal grains in the metal structure of the steel sheet. Accordingly, these elements are added as needed. When the total amount added of one or more members selected from Nb, Ti and V is lower than 0.01%, the action of Nb, Ti and v mentioned above is not effective. In contrast, when the above-mentioned amount added is higher than 0.1% in total, too much precipitate is produced, and stretch-flanging performance is thus significantly lowered. Therefore, the upper limit of the total amount added mentioned above is 0.1%.

The high strength steel sheet of this embodiment may be composed to contain Ni or Cu in an amount of 1% by mass or lower in place of Cr, Mo, Nb, Ti and V. Moreover, it may be composed to contain B in an amount of 0.0001% by mass or higher but 0.0010% by mass or lower. Further, it may be composed to contain 0.003% by mass or less of Ca and/or REM in total.

The material of the high strength steel sheet of this embodiment is composed of Fe and inevitable impurities, in addition to the above-mentioned components. P and S are present as inevitable impurities, but they do not adversely affect the characteristics of the high strength steel sheet of this embodiment as long as the amount of P is 0.05% or lower (not including 0%) and the amount of S is 0.02% or lower (including 0%). The less the amount of P and S contained, the better the processability of the steel sheet. Especially, when the amount of S contained is high, MnS which serves as a mediator is increased in steel, whereby the stretch-flanging performance of the steel sheet is significantly lowered.

Second, the metal structure of the high strength steel sheet of this embodiment will be described. The metal structure of the high strength steel sheet of this embodiment comprises a tempered martensite phase having a space factor of 80% or higher and a retained austenite phase having a space factor of 3% or lower, and the rest is mainly composed of a ferrite phase.

Among these constituent phases, the tempered martensite phase will be described first. When the space factor of the tempered martensite phase is 80% or higher, combination between austenite crystal grains and growth of the same can be suppressed by the annealed martensite phase remaining finely in part of the ferrite phase after the annealing step employed in the method for manufacturing the high strength steel sheet of the embodiment described later. When the space factor of the tempered martensite phase is lower than 80%, the tempered martensite phase is divided into ferrite phases, and therefore stretch-flanging performance is lowered. In contrast, when the phase becomes a substantially single-phase structure of tempered martensite having a space factor of the tempered martensite phase of 100%, ductility is lowered. For this reason, the case where the space factor is 100% is not included in the present invention.

In the tempered martensite phase of the high strength steel sheet of this embodiment, the mean grain size is 10 μm or smaller, and the space factor of the tempered martensite phase having a grain size larger than 10 μm is 15% or lower. When the mean grain size is larger than 10 μm, or when the space factor of the tempered martensite phase having a grain size larger than 10 μm is higher than 15%, the interfaces of the tempered martensite phase which act as the starting point of rupture are unevenly distributed, and therefore sufficient stretch-flanging performance cannot be obtained.

In the metal structure of the high strength steel sheet of this embodiment, the space factor of the retained austenite phase is 3% or lower. The retained austenite phase undergoes induced transformation in which it transforms into a tempered martensite phase during processing. Accordingly, the retained austenite phase lowers stretch-flanging performance. Therefore, in order to improve stretch-flanging performance, the space factor of the retained austenite phase needs to be limited to a low level. The space factor of the retained austenite phase is preferably 2% or lower, and more preferably 1% or lower.

The high strength steel sheet of the embodiment as described above has a fine tempered martensite phase formed therein and has a sufficiently low space factor of the retained austenite phase. Hence, it has excellent characteristics: it not only has high tensile strength, but also high elongation and stretch-flanging performance at the same time.

The method for manufacturing the high strength steel sheet of this embodiment will be now described.

First, materials of the high strength steel sheet of this embodiment will be described. The high strength steel sheet of this embodiment is obtained by subjecting a steel sheet material which meets predetermined conditions to a heat treatment comprising a predetermined annealing step and a tempering step.

The steel sheet material of the high strength steel sheet of this embodiment needs to meet the conditions of the composition of constituents mentioned above and also the conditions of the following metal structure. The steel sheet material of the high strength steel sheet of this embodiment needs to have a space factor of the martensite phase and the retained austenite phase of 90% or higher. Preferably, the space factor of the martensite phase and the retained austenite phase is 95% or higher. If the space factor of these constituent phases is lower than 90%, when the steel sheet is heated to a 2-phase region of the ferrite phase and austenite phase, a coarse austenite phase is produced. Therefore, the fine tempered martensite phase mentioned above cannot be obtained. This prevents improvement in stretch-flanging performance.

The steel sheet material of the high strength steel sheet of this embodiment having a space factor of the martensite phase and the retained austenite phase of 90% or higher is produced in the manner described below.

The process for manufacturing a steel sheet material of the high strength steel sheet of this embodiment (hereinafter referred to as “steel sheet material”) comprises the following steps: A steel slab adjusted to meet the composition of constituents of the high strength steel sheet material mentioned above is hot-rolled at such a temperature that the finishing rolling temperature is Ac₃ point or higher. this hot-rolled steel sheet is then cooled at a cooling rate of 10° C./sec. or higher to a cooling stop temperature, which is lower than Ms point at which the austenite phase starts to transform into the martensite phase (about 350° C. or lower), and is wound up. When the finishing rolling temperature is Ac₃ point or lower or the cooling rate after the hot rolling is 10° C./sec. or lower, the ferrite phase is likely to be produced during cooling after the hot rolling, and the space factor of the low-temperature transformation phase after the hot rolling does not become 90% or higher.

Even a steel sheet produced under conditions which do not meet those of the hot rolling and cooling rate mentioned above from a steel slab adjusted to meet the composition of constituents of the steel sheet material can be modified into a steel sheet material having a space factor of its low-temperature transformation phase of 90% or higher by carrying out the following preliminary annealing. This preliminary annealing is a heat treatment in which the hot-rolled steel sheet is retained in a temperature range of Ac₃ point or higher for 5 seconds or longer, and is cooled at a cooling rate of 10° C./sec. or higher to a cooling stop temperature of 350° C. or lower. When the steel sheet mentioned above is retained in a temperature range of Ac₃ point or lower, a ferrite phase is produced, and the space factor becomes no greater than 90%. Moreover, even when the steel sheet mentioned above is retained in a temperature range of Ac₃ point or higher, transformation of the metal structure into austenite is insufficient in case where the retaining time is shorter than 5 seconds, and the space factor thus becomes no greater than 90%. As long as the conditions of this preliminary annealing are met, the upper limit of the annealing temperature, retaining time and cooling rate and the lower limit of the cooling stop temperature are not specifically determined.

The heat treatment step of the high strength steel sheet of this embodiment will be described now. The high strength steel sheet of this embodiment is obtained by subjecting a steel sheet material to a heat treatment comprising a predetermined annealing step and a tempering step. This annealing step is a heat treatment in which the steel sheet material is heated to a temperature of Ac₃ point or lower but not lower than Ac₃ point −50°, retained for 30 seconds or longer but not longer than 1200 seconds, and is then cooled at a cooling rate of 10° C./sec. or higher to Ms point or lower. By conducting this annealing step, the above-mentioned martensite phase having a space factor of 80% or higher is formed. Moreover, the size of austenite crystal grains produced when the steel sheet material is heated to and retained at a temperature of Ac₃ point or lower but not lower than Ac₃ point −50% affects the crystal grain size of the tempered martensite phase of the high strength steel sheet of the embodiment. That is, to obtain a fine tempered martensite having a mean grain size of 10 μm or smaller and the space factor of the tempered martensite phase having a grain size larger than 10 μm is 15% or lower phase as the high strength steel sheet of this embodiment, the steel sheet material needs to be heated to and retained at a temperature of Ac₃ point or lower but not lower than Ac₃ point −50°. A steel sheet having the metal structure in which such a fine tempered martensite phase is formed is characterized by high strength and high ductility.

In this annealing step, when the steel sheet material is retained in a temperature range higher than Ac₃ point at which the austenite single-phase is stable, crystal grains of austenite grow and combine with each other to be coarse. Therefore, the steel sheet material cannot be imparted a metal structure having a fine tempered martensite phase as the high strength steel sheet of this embodiment. As a result, the stretch-flanging performance of the high strength steel sheet is lowered. Moreover, when the steel sheet material is retained at a temperature lower than Ac₃ point −50° C., transformation into austenite does not proceed sufficiently, and the space factor of the tempered martensite phase of the high strength steel sheet after the heat treatment becomes lower than that of the high strength steel sheet of this embodiment. As a result, the stretch-flanging performance of the high strength steel sheet is lowered. Therefore, the retaining temperature was set to Ac₃ point or lower but not lower than Ac₃ point −50° C.

When the retaining time in this annealing step is shorter than 30 seconds, the austenite phase is not sufficiently produced, and thus a fine martensite phase cannot be obtained after this annealing step. When the retaining time is longer than 1200 seconds, produced austenite crystal grains become coarse, and therefore the fine tempered martensite phase mentioned above cannot be obtained. Accordingly, the retaining time is to be in the range of 30 seconds or longer but not longer than 1200 seconds, Preferably, it is in the range of 120 seconds or longer but not longer than 600 seconds.

In this annealing step, when the cooling rate is 10° C./sec. or lower, or the cooling stop temperature is higher than Ms point at which the transformation from the austenite phase into the tempered martensite phase starts, generation of a bainite phase, retained austenite phase, pearlite phase and ferrite phase and deposition of a cementite phase are caused, and a number of phases other than the martensite phase are formed, whereby the space factor of the martensite phase cannot be increased. Accordingly, the stretch-flanging performance of the steel sheet is lowered. The higher the cooling rate, and the lower the cooling stop temperature, the higher the space factor of the tempered martensite phase can be.

The tempering step will be described now. The steel sheet material which has undergone the annealing step is retained at a temperature of 300° C. to 550° C. for 60 seconds to 1200 seconds. A fine martensite phase is formed in the metal structure of the steel sheet material which has undergone the annealing step. The steel sheet material is softened by tempering this martensite phase to reduce a difference in hardness from the annealed martensite phase and ferrite phase, whereby excellent stretch-flanging performance, as well as ductility, can be obtained.

When the retaining temperature in this tempering step is lower than 300° C., the hardness of the tempered martensite phase is too high, and the stretch-flanging performance of the steel sheet is thus lowered. In contrast, when the retaining temperature is higher than 550° C., the cementite phase produced by the decomposition of the retained austenite phase becomes coarse, whereby the stretch-flanging performance of the steel sheet is lowered.

When the retaining time in this tempering step is shorter than 60 seconds, the hardness of the tempered martensite phase is too high, and therefore the elongation and stretch-flanging performance of the steel sheet are lowered. In contrast, When the retaining time is longer than 1200 seconds, the cementite phase become coarse, and the stretch-flanging performance of the steel sheet is lowered. The retaining time in this tempering step is 60 seconds or longer but not longer than 1200 seconds, but it is preferably 90 seconds or longer but not longer than 900 seconds, and more preferably 120 seconds or longer but not longer than 600 seconds.

The steel sheet material which has been subjected to the annealing step and this tempering step becomes the high strength steel sheet of this embodiment, and is characterized by high stretch-flanging performance, in addition to high tensile strength and high ductility. Accordingly, this high strength steel sheet is used for various industrial products typically including automobiles as a steel sheet having excellent press formability.

Example 2

The actions and effects of the high strength steel sheet of this embodiment and method for manufacturing the same will be described below with reference to Examples.

First, a method for preparing test steel sheets tested in these Examples will be described. In these Examples, Steel slabs having the compositions of constituents represented by steel symbols A to Y having the compositions of constituents shown in Table 5 were tested. As shown in Table 6 and 7, 56 types of test steel sheets were prepared under different hot rolling conditions, preliminary annealing conditions, and in different annealing steps and tempering steps from the steel slabs having the compositions of constituents of these A to Y, and the tensile strength, ductility, stretch-flanging performance and other characteristics of the test steel sheets were determined. Among the steel slabs having the compositions of constituents of A to Y, B, C, E, F, I, J, L, N to Y are the steel slabs having the compositions of constituents which fall within the Examples of the embodiment. Steel slabs having other compositions of constituents do not fall within the compositions of constituents of this embodiment. As can be seen from Tables 6 and 7, the test steel sheets prepared from these steel slabs are Comparative Examples. The steel slabs having the compositions of constituents of these A to Y, respectively, were hot-rolled at a finishing temperature of 850° C. to give 56 types of test steel sheets having a thickness of 3 mm (Nos. 1 to 56), which were then wound up at predetermined temperatures shown in Table 6. Furthermore, the test steel sheets No. 1 to 45 were washed with acid to remove scales, and were cold-rolled to a thickness of 1.2 mm. The test steel sheets excluding test steel sheets 2 and 11 were then subjected to preliminary annealing under predetermined conditions shown in Table 6. Thereafter, test steel sheets Nos. 1 to 56 were subjected to the heat treatment comprising the annealing step and the tempering step under predetermined conditions shown in Table 7, and were used as test steel sheets for measurement.

TABLE 5 Transformation Steel Chemical components (% by mass) temperature (° C.) symbol C Si Mn P S Al Mo Cr Ti Nb V Others Ac₃ Mg Remarks A 0.02 1.52 1.90 0.012 0.002 0.027 911 489 Comparative example B 0.06 1.51 1.93 0.003 0.001 0.026 882 469 Example of invention C 0.22 1.51 2.05 0.006 0.002 0.031 837 389 Example of invention D 0.35 1.49 1.98 0.005 0.002 0.032 813 330 Comparative example E 0.18 0.05 2.03 0.008 0.001 0.029 782 409 Example of invention F 0.16 2.88 2.05 0.008 0.002 0.033 915 418 Example of invention G 0.21 3.25 2.08 0.018 0.001 0.028 924 393 Comparative example H 0.19 1.51 0.41 0.009 0.003 0.026 893 457 Comparative example I 0.20 1.49 0.56 0.009 0.001 0.028 887 448 Example of invention J 0.22 1.48 2.95 0.004 0.002 0.031 908 359 Example of invention K 0.19 1.50 3.25 0.007 0.001 0.033 809 354 Comparative example L 0.21 1.48 1.94 0.018 0.002 0.088 871 397 Example of invention M 0.20 1.49 1.95 0.009 0.003 0.110 877 401 Comparative example N 0.18 1.50 2.08 0.015 0.001 0.031 0.20 858 403 Example of invention O 0.19 1.52 1.92 0.012 0.002 0.026 0.10 0.20 852 402 Example of invention P 0.22 1.51 2.95 0.005 0.002 0.027 0.05 828 358 Example of invention Q 0.21 1.51 2.05 0.006 0.002 0.028 0.025 838 394 Example of invention R 0.20 1.48 1.90 0.006 0.001 0.031 0.10 855 404 Example of invention S 0.19 1.52 2.10 0.008 0.001 0.031 0.05 0.05 870 402 Example of invention T 0.20 1.47 2.04 0.015 0.003 0.027 0.024 0.05 850 398 Example of invention U 0.18 1.49 1.99 0.009 0.002 0.048 856 409 Example of invention V 0.17 1.43 2.03 0.011 0.003 0.051 Ni: 0.2 854 408 Example of invention W 0.16 1.52 1.86 0.012 0.003 0.047 Cu: 0.1 863 416 Example of invention X 0.17 1.49 2.11 0.009 0.002 0.040 B: 0.003 851 408 Example of invention Y 0.18 1.38 1.89 0.011 0.002 0.045 Ca + REM: 0.001 855 413 Example of invention (Note) Remainder is Fe and inevitable impurities

As can be seen from Table 6, the steel sheets corresponding to Examples all have a space factor of the low-temperature transformation phase of 90% or higher, which falls within the conditions of the steel sheet material.

TABLE 6 Structure before annealing Hot-rolling Space Test conditions Preliminary annealing conditions factor of steel Finishing Winding Heating Retaining Cooling stop M + sheet Steel temperature temperature temperature time Cooling rate temperature Phase retained γ No. symbol (° C.) (° C.) (° C.) (sec.) (° C./sec.) (° C.) constitution (%) Remarks 1 A 850 500 930 120 600 20 M 100 Comparative example 2 B 850 500 — — — — α + M 85 Comparative example 3 B 850 500 930 120 600 20 M 100 Example of invention 4 C 850 500 800 120 600 20 α + M 80 Comparative example 5 C 850 500 930 120 600 20 M 100 Example of invention 6 C 850 500 930  2 600 20 α + M 80 Comparative example 7 C 850 500 930  10 600 20 α + M 90 Example of invention 8 C 850 500 930 600 600 20 M 100 Example of invention 9 C 850 500 930 120  5 20 α + M 70 Comparative example 10 C 850 500 930 120 600 500  B 0 Comparative example 11 C 850 300 — — — — α + M 95 Example of invention 12 D 850 500 930 120 600 20 M 100 Comparative example 13 E 850 500 930 120 600 20 M 100 Example of invention 14 F 850 500 930 120 600 20 M 100 Example of invention 15 G 850 500 930 120 600 20 M 100 Comparative example 16 H 850 500 930 120 600 20 M 100 Comparative example 17 I 850 500 930 120 600 20 M 100 Example of invention 18 J 850 500 930 120 600 20 M 100 Example of invention 19 K 850 500 930 120 600 20 M 100 Comparative example 20 L 850 500 930 120 600 20 M 100 Example of invention 21 M 850 500 930 120 600 20 M 100 Comparative example 22 N 850 500 930 120 600 20 M 100 Comparative example 23 N 850 500 930 120 600 20 M 100 Example of invention 24 N 850 500 930 120 600 20 M 100 Example of invention 25 N 850 500 930 120 600 20 M 100 Comparative example 26 N 850 500 930 120 600 20 M 100 Comparative example 27 N 850 500 930 120 600 20 M 100 Example of invention 28 N 850 500 930 120 600 20 M 100 Example of invention 29 N 850 500 930 120 600 20 M 100 Comparative example 30 N 850 500 930 120 600 20 M 100 Comparative example 31 N 850 500 930 120 600 20 M 100 Comparative example 32 N 850 500 930 120 600 20 M 100 Comparative example 33 N 850 500 930 120 600 20 M 100 Example of invention 34 N 850 500 930 120 600 20 M 100 Example of invention 35 N 850 500 930 120 600 20 M 100 Comparative example 36 N 850 500 930 120 600 20 M 100 Comparative example 37 N 850 500 930 120 600 20 M 100 Example of invention 38 N 850 500 930 120 600 20 M 100 Example of invention 39 N 850 500 930 120 600 20 M 100 Comparative example 40 O 850 500 930 120 600 20 M 100 Example of invention 41 P 850 500 930 120 600 20 M 100 Example of invention 42 Q 850 500 930 120 600 20 M 100 Example of invention 43 R 850 500 930 120 600 20 M 100 Example of invention 44 S 850 500 930 120 600 20 M 100 Example of invention 45 T 850 500 930 120 600 20 M 100 Example of invention 46 U 900 500 930 180 700 20 M 100 Example of invention 47 V 900 500 930 120 700 20 M 100 Example of invention 48 W 900 500 930 120 700 20 M 100 Example of invention 49 X 900 500 930 180 700 20 M 100 Example of invention 50 Y 900 500 930 240 700 20 M 100 Example of invention 51 U 900 250 — — — — M 100 Example of invention 52 U 950 300 — — — — M 100 Example of invention 53 U 900 200 — — — — M 100 Example of invention 54 V 920 280 — — — — M 100 Example of invention 55 W 920 300 — — — — M 100 Example of invention 56 X 920 300 — — — — M 100 Example of invention (Note): M: Martensite phase, α: Ferrite phase, B: Bainite phase, retained γ: Retained austenite phase

As shown in Table 7, among the 56 types of test steel sheets prepared, 33 types correspond to Examples corresponding to the present embodiment, while others are Comparative Examples.

TABLE 7 Tempering Test Annealing conditions conditions steel Heating Retaining Cooling Cooling stop Heating Retaining sheet Steel temperature time rate temperature temperature time No. symbol (° C.) (sec.) (° C./sec.) (° C.) (° C.) (sec.) Remarks 1 A 880 180 600 20 450 180 Comparative example 2 B 850 180 600 20 450 180 Comparative example 3 B 850 180 600 20 450 180 Example of invention 4 C 800 180 600 20 450 180 Comparative example 5 C 800 180 600 20 450 180 Example of invention 6 C 800 180 600 20 450 180 Comparative example 7 C 800 180 600 20 450 180 Example of invention 8 C 800 180 600 20 450 180 Example of invention 9 C 800 180 600 20 450 180 Comparative example 10 C 800 180 600 20 450 180 Comparative example 11 C 800 180 600 20 450 180 Example of invention 12 D 780 180 600 20 450 180 Comparative example 13 E 750 180 600 20 450 180 Example of invention 14 F 860 180 600 20 450 180 Example of invention 15 G 860 180 600 20 450 180 Comparative example 16 H 850 180 600 20 450 180 Comparative example 17 I 850 180 600 20 450 180 Example of invention 18 J 780 180 600 20 450 180 Example of invention 19 K 780 180 600 20 450 180 Comparative example 20 L 850 180 600 20 450 180 Example of invention 21 M 850 180 600 20 450 180 Comparative example 22 N 900 180 600 20 450 180 Comparative example 23 N 840 180 600 20 450 180 Example of invention 24 N 820 180 600 20 450 180 Example of invention 25 N 780 180 600 20 450 180 Comparative example 26 N 820 20 600 20 450 180 Comparative example 27 N 820 50 600 20 450 180 Example of invention 28 N 820 1000 600 20 450 180 Example of invention 29 N 820 1500 600 20 450 180 Comparative example 30 N 820 180 5 20 450 180 Comparative example 31 N 820 180 600 500 450 180 Comparative example 32 N 820 180 600 20 250 180 Comparative example 33 N 820 180 600 20 350 180 Example of invention 34 N 820 180 600 20 550 180 Example of invention 35 N 820 180 600 20 650 180 Comparative example 36 N 820 180 600 20 450 40 Comparative example 37 N 820 180 600 20 450 50 Example of invention 38 N 820 180 600 20 450 1000 Example of invention 39 N 820 180 600 20 450 1500 Comparative example 40 O 820 180 600 20 450 180 Example of invention 41 P 800 180 600 20 450 180 Example of invention 42 Q 800 180 600 20 450 180 Example of invention 43 R 820 180 600 20 450 180 Example of invention 44 S 850 180 600 20 450 180 Example of invention 45 T 820 180 600 20 450 180 Example of invention 46 U 840 180 700 20 500 180 Example of invention 47 V 840 120 700 20 500 180 Example of invention 48 W 850 180 300 20 400 120 Example of invention 49 X 820 120 700 20 500 180 Example of invention 50 Y 820 180 500 20 450 120 Example of invention 51 U 840 120 700 20 500 180 Example of invention 52 U 820 240 500 50 500 240 Example of invention 53 U 820 180 500 20 500 180 Example of invention 54 V 840 120 500 20 400 180 Example of invention 55 W 850 180 500 20 400 180 Example of invention 56 X 820 120 500 20 400 180 Example of invention

The 56 types of the test steel sheets prepared by these steps were tested for their tensile strength and stretch-flanging performance.

The tensile strength test was performed in such a manner that the direction perpendicular to the rolling direction of each of the test steel sheets is the direction of pulling during testing by using a JIS No. 5 test piece collected from each test steel sheet according to JIS Z2241. In this test, yield strength YS, tensile strength TS and elongation EL were determined.

The stretch-flanging performance test was performed according to Japan Iron and Steel Federation standard (JFST 1001-1996), and hole expansion rates λ were determined.

The measurement results of the characteristics of the 56 types of test steel sheets are shown in Tables 8 and 9. In Tables 8 and 9, AM represents an annealed martensite phase; TM represents a tempered martensite phase; and retained γ represents a retained austenite phase. The space factor of the retained austenite phase was indicated 0% when it was below the detection limit.

In the results of this test, when the tensile strength is 780 MPa or higher, it is a sufficient strength characteristic in practical use, and meets the conditions of the tensile strength of the present invention. Moreover, elongation (ductility) and stretch-flanging performance are considered excellent when they are 10% or higher and 80% or higher, respectively. The stretch-flanging performance is judged especially excellent characteristics when it is 100% or higher.

Test steel sheets which meet all the following conditions are considered to correspond to high strength steel sheets according to the present invention: tensile strength: TS≧780 MPa, elongation: EL≧10%, hole expansion rate: λ≧80%. A test steel sheet which meets all these three conditions and has especially good hole expansion rate (λ≧100%) was rated ⊚; a test steel sheet which meets all the conditions was rated ◯; a test steel sheet which meets two conditions out of three was rated Δ; and a test steel sheet which meets only one condition or less out of three conditions was rated x.

TABLE 8 Metal structure parameters Space factor of TM having Test Mean Space Space grain size steel grain size factor factor of of 10 μm Dynamic characteristics sheet Steel of TM of TM retained γ or larger YS TS EL λ No. symbol (μm) (%) (%) (%) (MPa) (MPa) (%) (%) Judgment Remarks 1 A 6.8 89 0 8 365 608 32.2 70.1 X Comparative example 2 B 8.8 87 0 18 523 752 28.7 82.7 Δ Comparative example 3 B 7.0 89 0 7 598 801 26.5 122.2 ⊚ Example of invention 4 C 9.2 88 0 18 961 1011 8.5 61.2 X Comparative example 5 C 6.2 88 0 11 953 1052 11.6 105.2 ⊚ Example of invention 6 C 11.4 88 0 20 811 1084 7.6 45.4 X Comparative example 7 C 9.7 88 0 12 862 1098 12.1 83.4 ◯ Example of invention 8 C 7.0 88 0 10 873 1065 12.7 103.8 ⊚ Example of invention 9 C 9.8 88 0 18 795 1035 8.9 65.2 X Comparative example 10 C 11.0 88 0 20 888 1069 8.6 75.8 X Comparative example 11 C 8.4 88 0 12 996 1127 12 98.9 ◯ Example of invention 12 D 6.6 89 0 7 887 1220 11.2 76.2 Δ Comparative example 13 E 7.6 89 0 11 776 967 13.9 90.5 ◯ Example of invention 14 F 6.6 89 0 8 898 1263 15.8 114.2 ⊚ Example of invention 15 G 8.0 87 0 8 1091 1318 10.3 67.7 Δ Comparative example 16 H 7.2 87 6 11 832 1011 8.3 46.2 X Comparative example 17 I 6.2 86 3 6 963 1081 10.3 89.1 ◯ Example of invention 18 J 7.4 90 0 8 1011 1122 11.3 81.1 ◯ Example of invention 19 K 6.8 90 0 11 1194 1271 7.3 48.3 X Comparative example 20 L 7.6 91 0 7 785 823 34.7 106.4 ⊚ Example of invention 21 M 8.2 78 0 10 763 845 36.2 65.4 Δ Comparative example 22 N 12.7 100 0 62 1120 1265 8.1 81.7 Δ Comparative example 23 N 7.0 92 0 13 996 1063 12.2 88.3 ◯ Example of invention 24 N 8.6 88 0 11 902 1003 15.7 86.6 ◯ Example of invention 25 N 8.4 72 0 12 855 980 17.8 58.7 Δ Comparative example 26 N 7.0 62 0 9 913 969 15.2 81.7 Δ Comparative example 27 N 8.0 81 0 10 909 1029 12.2 91.5 ◯ Example of invention 28 N 9.4 86 0 13 1056 1101 10.4 85.4 ◯ Example of invention 29 N 11.7 88 0 24 1101 1248 7.2 96.0 Δ Comparative example 30 N 8.0 72 0 7 878 758 16.7 80.2 Δ Comparative example 31 N 8.2 23 8 10 798 856 20.2 34.4 Δ Comparative example 32 N 7.2 88 6 5 1229 1499 8.2 39.3 X Comparative example 33 N 7.4 88 2 6 1129 1225 10.2 83.8 ◯ Example of invention 34 N 7.2 88 0 12 1076 1158 11.4 89.2 ◯ Example of invention 35 N 9.6 88 0 16 587 778 16.7 45.0 Δ Comparative example 36 N 7.0 88 7 6 826 1205 12.3 40.6 Δ Comparative example 37 N 6.9 88 3 7 972 1258 10.3 85.6 ◯ Example of invention 38 N 7.2 86 0 7 1023 1181 11.5 91.3 ◯ Example of invention 39 N 10.2 88 0 17 606 798 16.5 51.6 Δ Comparative example 40 O 6.2 89 0 7 1014 1150 12.3 86.5 ◯ Example of invention 41 P 6.4 90 0 6 987 1097 11.9 82.2 ◯ Example of invention 42 Q 6.2 88 0 6 960 983 10.3 98.3 ◯ Example of invention 43 R 6.4 88 0 8 936 952 10.9 86.2 ◯ Example of invention 44 S 6.6 92 0 8 1023 1095 10.4 88.5 ◯ Example of invention 45 T 6.3 89 0 7 1008 1109 13.3 83.4 ◯ Example of invention

TABLE 9 Metal structure parameters Space factor of TM having Test Space Mean Space Space grain size Dynamic steel factor grain size factor factor of of 10 μm characteristics sheet Steel of AM of TM of TM retained γ or larger TS EL λ No. symbol (%) (μm) (%) (%) (%) (MPa) (%) (%) Judgment Remarks 46 a 11 8.3 89 0 10 813 31.5 108.3 ⊚ Example of invention 47 b 10 8.5 90 0 9 837 30.8 98.9 ◯ Example of invention 48 c 10 8.6 80 0 8 870 28.3 92.1 ◯ Example of invention 49 d 15 7.2 85 0 12 860 28.3 93.7 ◯ Example of invention 50 e 16 7.4 84 0 11 887 21.9 97.5 ◯ Example of invention 51 e 11 8.3 89 0 9 844 28.9 92.4 ◯ Example of invention 52 a 16 7.4 85 0 13 825 30.1 89.3 ◯ Example of invention 53 a 16 7.6 84 0 13 838 26.9 96.6 ◯ Example of invention 54 b 9 8.1 91 0 7 913 17.3 115.0 ⊚ Example of invention 55 c 10 8.4 90 0 8 942 15.3 109.3 ⊚ Example of invention 56 d 14 7.5 86 0 11 938 14.9 107.4 ⊚ Example of invention

The results of the measurement of test steel sheet characteristics in Table 8 will be described below.

The test steel sheets No. 3, 5, 7, 8, 11, 13, 14, 17, 18, 20, 23, 24, 27, 28, 33, 34, 37, 38 and 40 to 45 are all prepared from steel slabs (B, C, E, F, I, J, L, N to T in Table 5) corresponding to the compositions of constituents of the high strength steel sheet of this embodiment. Moreover, as can be seen from Tables 6 and 7, the space factor of the martensite phase and the retained austenite phase of the metal structures of these test steel sheets before the annealing step, the annealing step and the tempering step correspond to the conditions of the high strength steel sheet of this embodiment. All of these test steel sheets meet the conditions of the tensile strength, elongation and stretch-flanging performance of the present invention.

The test steel sheets (Nos. 46 to 56) of Table 9 all meet the conditions of the tensile strength, elongation and stretch-flanging performance of the present invention.

Among the test steel sheets which correspond to the high strength steel sheet of the embodiment, Nos. 3, 5, 8, 14 and 20 have especially good stretch-flanging performance. The space factor of the retained austenite phase of these test steel sheets is 0%. The tempered martensite phases of these test steel sheets have relatively small mean grain sizes, and the space factor of the tempered martensite phase having a crystal grain size of 10 μm or larger is relatively low.

The reason why the test steel sheets of Comparative Examples did not meet the conditions of the high strength steel sheet according to the present invention will be described.

Since the test steel sheet No. 1 was prepared from steel slab A having low level of C, it has low tensile strength.

Test steel sheet No. 2 had low space factors of the martensite phase and the retained austenite phase in the metal structure in a state that it is yet to be annealed were low. Therefore, crystal grains of the tempered martensite phase became coarse, and the strength and stretch-flanging performance were lowered.

Test steel sheet No. 4 was subjected to the preliminary annealing at a temperature lower than Ac₃ point, and therefore the space factor of the low-temperature transformation phase in the metal structure in a state that it is yet to be annealed was lowered. This caused crystal grains of the tempered martensite phase to be coarse, whereby ductility and stretch-flanging performance are low.

The test steel sheet No. 6 had low space factors of the martensite phase and the retained austenite phase in the metal structure in a state that it was yet to be annealed because the retaining time in the preliminary annealing was short, and therefore crystal grains in the tempered martensite phase became coarse. As a result, it had low elongation and stretch-flanging performance.

The test steel sheet No. 9 had low space factors of the martensite phase and the retained austenite phase in the metal structure in a state that it was yet to be annealed because cooling after the preliminary annealing was delayed, and therefore the tempered martensite phase became coarse. As a result, it had low elongation and stretch-flanging performance.

The test steel sheet No. 10 had low space factors of the martensite phase and the retained austenite phase in the metal structure in a state that it was yet to be annealed because the cooling stop temperature after the preliminary annealing was high, and therefore the tempered martensite phase became coarse. As a result, it had low elongation and stretch-flanging performance.

Although the test steel sheet No. 12 has a metal structure after the tempering step corresponding to that of the high strength steel sheet of the embodiment, the difference in strength between the annealed martensite phase which is a part of the ferrite phase and the tempered martensite phase has not been sufficiently reduced because this test steel sheet was prepared from steel slab D having a high level of C. As a result, it had low stretch-flanging performance.

Although the test steel sheet No. 15 has a metal structure after the tempering step corresponding to that of the high strength steel sheet of the embodiment, this test steel sheet was prepared from steel slab G having a high level of Si. Accordingly, the tempered martensite phase was not sufficiently tempered, and the difference in strength between the annealed martensite phase which is a part of the ferrite phase and the tempered martensite phase has not been sufficiently reduced. As a result, it had low stretch-flanging performance.

Since the test steel sheet No. 16 was prepared from steel slab H having a low level of Mn, it has insufficient hardening characteristics, and therefore a large amount of retained austenite remained after the annealing step. As a result, it had low elongation and stretch-flanging performance.

Since the test steel sheet No. 19 was prepared from steel slab K having a high level of Mn, uneven distribution of Mn occurred although the space factors and the size of the martensite phase and the retained austenite phase in the metal structure after the tempering step correspond to that of the high strength steel sheet of the embodiment. As a result, it had low elongation and stretch-flanging performance.

The test steel sheet No. 21 was prepared from steel slab M having a high amount of Al added. Accordingly, it had a number of flaws on the surface of the steel material. As a result, it had low stretch-flanging performance.

The test steel sheet No. 22 had coarse crystal grains of the austenite phase since it was heated to a temperature higher than Ac₃ point in the annealing step. As a result, its ductility was lowered.

In the test steel sheet No. 25, the austenite phase was not sufficiently produced because the heating and retaining temperature in the annealing step was lower than Ac₃ point −50° C. As a result, it had low space factor of the tempered martensite phase, and low stretch-flanging performance.

In the test steel sheet No. 26, the austenite phase was not sufficiently produced because the retaining time at a temperature of Ac₃ point or lower but not higher than Ac₃ point −50° C. in the annealing step was too short. As a result, it had low space factor of the martensite phase and low stretch-flanging performance.

In the test steel sheet No. 29, the crystal grains of the austenite phase became coarse because the retaining time at a temperature of Ac₃ point or lower but not higher than Ac₃ point −50° C. in the annealing step was too long. As a result, it had large crystal grain size of the martensite phase, and low ductility.

In the test steel sheet No. 30, the tempered martensite phase was not sufficiently generated because cooling after the annealing step was too late and thus tempered phases other than the martensite phase were produced. As a result, it had low tensile strength.

In the test steel sheet No. 31, the martensite phase was not sufficiently produced because the cooling stop temperature after the annealing step was higher than Ms point. As a result, it had low space factor of the tempered martensite phase, and low stretch-flanging performance.

In the test steel sheet No. 32, the dislocation density of the tempered martensite phase was not lowered because the heating and retaining temperature in the tempering step was lower than the lower limit value, and distortion was not sufficiently mitigated. As a result, it had low elongation and stretch-flanging performance.

In the test steel sheet No. 35, cementite was deposited because the heating and retaining temperature in the tempering step was higher than the upper limit value. As a result, it had low stretch-flanging performance.

In the test steel sheet No. 36, the space factor of the retained austenite phase was not sufficiently lowered because the heating and retaining time in the tempering step was too short. Moreover, the dislocation density of the tempered martensite phase was not lowered, and distortion was not sufficiently mitigated. As a result, it had low stretch-flanging performance.

In the test steel sheet No. 39, cementite was deposited because the heating and retaining time in the tempering step was too long. As a result, it had low stretch-flanging performance.

(3)

Still another embodiment of the present invention will be described in detail below.

The inventors of the present invention studied the requirements for obtaining both strength and elongation, which are the features of a dual phase steel sheet (DP steel sheet), and also stretch-flanging performance by presupposingly using this DP steel sheet comprising the ferrite phase and martensite, from various angles. As a result, the inventors of the present invention found that a very fine ferrite+martensite dual structure can be obtained by subjecting a steel sheet having a fine lath-shaped structure (martensite and/or bainite) as a material steel sheet (that is, as an initial structure) to annealing (hereinafter referred to as “dual-phase range annealing”) in a dual-phase range (ferrite+austenite range). Moreover, the inventors of the present invention also found that a steel sheet having such a structure has good elongation and stretch-flanging performance.

In a steel sheet having the fine lath-shaped structure (martensite and/or bainite) as mentioned above, ferrite produced by the dual-phase range annealing is finely dispersed, and the growth of austenite during the dual-phase range annealing is suppressed by its pinning effect. Accordingly, its structure after hardening becomes a very fine ferrite+martensite structure. In addition, crystal grain micronizing elements such as Ti, Nb, V and Zr are added to the steel sheet as chemical components, whereby further micronization of the structure can be achieved. The thus-obtained dual phase steel sheet is imparted further improved elongation and stretch-flanging performance.

The high strength steel sheet of the present invention is a dual phase steel sheet which is mainly composed of a ferrite phase and martensite. In order to achieve the above object, it is necessary that the space factors of these phases to the entire structure are adjusted appropriately. That is, in the high strength steel sheet of the present invention, the space factors of the ferrite phase and martensite are 5 to 30% and 50 to 95%, respectively.

When the space factor of the ferrite phase is 5% or lower, good elongation cannot be ensured, and the pinning effect which suppresses the growth of austenite becomes weak. When the space factor is higher than 30%, the stretch-flanging performance is deteriorated. A preferable space factor of the ferrite phase is 7% or higher but not higher than 25%.

When the space factor of martensite is lower than 50%, the stretch-flanging performance is lowered. When the space factor is higher than 95%, elongation is lowered. A preferable space factor of the martensite phase is 70% or higher but not higher than 85%.

It should be noted that the term “space factor” mentioned above means the ratio (% by volume) of each phase constituting a metal structure in the steel material to the entire structure, and the space factors of the ferrite phase and martensite can be determined by corroding a steel material with nital, observing the material with an optical microscope (1000 times), and then subjecting the material to image analysis.

In the high strength steel sheet of the present invention, it is preferable that the mean grain size of the above ferrite phase is 3 μm or smaller in terms of the equivalent of a circle diameter, and that the mean grain size of the martensite phase is 6 μm or smaller in terms of the equivalent of a circle diameter. If these sizes are increased, elongation and stretch-flanging performance are lowered. The “mean grain sizes” of these phases are determined, for example, by measuring the grain sizes of twenty grains by observing the structure using an optical microscope and FE/SEM-EBSP, and averaging the measurements.

The dual phase steel sheet according to the present invention is composed of a ferrite phase and martensite as its main structure, but it is not necessarily 100% composed of these phases, and it is also allowed that at least the total sum is 70% or higher, preferably 80% or higher, in terms of space factor, due to the intention that it is merely the main part, and that bainite, pearlite, retained austenite and the like are contained. However, the less these structures, the better, from the standpoint of not reducing stretch-flanging performance.

In the steel sheet of the present invention, the structure is controlled in the above-mentioned manner, whereby good elongation and stretch-flanging performance are exhibited. A preferable composition of constituents considering the strength (590 MPa or higher as tensile strength TS) and other points is as follows: C: 0.05 to 0.3%; Si: 0.01 to 3%; Mn: 0.5 to 3.0%; Al: 0.01 to 0.1%; at least one element selected from the group consisting of Ti, Nb, V and Zr: 0.01 to 1% in total; and iron and inevitable impurities as the remainder. The reason for the definition of these preferable ranges is as follow:

[C: 0.05 to 0.3%]

C is an important element in producing martensite to increase the strength of the steel sheet. To perform such an effect, the amount of C contained is preferably 0.05% or higher. From the perspective of increasing strength, the higher the amount of C contained, the better. However, if the amount of C is excessively high, a large amount of retained austenite which deteriorates stretch-flanging performance is produced, and weldability is also adversely affected. Therefore, the amount is preferably 0.3% or lower, A more preferable lower limit of the amount of C contained is 0.07%, and a more preferable upper limit is 0.25%.

[Si: 0.01 to 3%]

Si is an element which effectively acts as a deoxidizing element when steel is melted, and effectively increases strength without deteriorating the ductility of steel. It also acts to suppress deposition of coarse carbide which deteriorates stretch-flanging performance. In order to perform these effects effectively, it is preferably contained in an amount of 0.01% or higher. However, since the effect of adding Si is saturated in an amount of about 3%, A preferable upper limit is set to 3%. A more preferable lower limit of the amount of Si contained is 0.1%, and a more preferable upper limit is 2.5%.

[Mn: 0.5 to 3.0%]

Mn is an element useful in increasing the hardening characteristics of the steel sheet to ensure high strength. To perform such an effect, it is preferably contained in an amount of 0.5% or higher. However, when the amount of Mn contained is excessively high, ductility is lowered and therefore processability is adversely affected. For this reason, the upper limit is set to 3.0%. A more preferable amount of Mn contained is 0.7% or higher but not higher than 2.5% or lower.

[Al: 0.01 to 0.1%]

Al is an element having a deoxidation effect, and when Al deoxidation is performed, it needs to be added in an amount of 0.01% or higher. However, when the amount of Al contained is excessively high, the above effect is saturated, and it also becomes a source of non-metallic mediators to deteriorate physical properties and surface properties. Therefore, the upper limit is set to 0.1%. a more preferable amount of Al contained is 0.03% or higher but not higher than 0.08%.

[One or more members selected from the group consisting of Ti, Nb, V and Zr: 0.01 to 1% in total]

These elements have the action to form precipitates such as carbide, nitride and carbonitride together with C and N to contribute to increased strength, and micronize crystal grains during hot rolling to increase elongation and stretch-flanging performance. Such effects are effectively performed when they are added in an amount of 0.01% or higher in total (of one or more members). A more preferable amount of these elements contained is 0.03% or higher. however, when the amount is excessively high, elongation and stretch-flanging performance are deteriorated rather than improved. Therefore, the amount is to be limited to 1% or lower, and more preferably 0.7% or lower.

Preferable basic components in the dual phase steel sheet of the present invention are as stated above, and the remainder is iron and inevitable impurities. Examples of inevitable impurities include steel raw materials, and P, S, N, O and others which can get into steel during the manufacturing process of the materials, among others.

It is also effective to add to the steel sheet of the present invention, as needed, the following substances: (a) Ni and/or Cu: 1% or lower (not including 0%) in total; (b) Cr: 2% or lower (not including 0%) and/or Mo: 1% or lower (not including 0%); (c) B: 0.0001 to 0.005%; and (d) Ca and/or REM: 0.003% or lower (not including 0%) in total, among others. The characteristics of the steel sheet are further improved depending on the types of the components contained. The reason for setting the range of these elements when contained is as follows.

[Ni and/or Cu: 1% or Lower (not Including 0%) in Total]

These elements are effective in maintaining the balance of strength and ductility high and realizing high strength at the same time. such effects increase as their amount contained is increased, but if they are contained in an amount higher than 1% in total (of one or more members), the above effects may be saturated and cracks may be produced during hot rolling. A more preferable lower limit of the amount of these elements contained is 0.05%, and a more preferable upper limit is 0.7%.

[Cr: 2% or Lower (not Including 0%) and/or Mo: 1% or Lower (not Including 0%)]

Both Cr and Mo are elements effective in stabilizing the austenite phase, and facilitating the generation of the low-temperature transformation phase in the course of cooling. Although their effects increase as their amount contained is increased, if they are contained in an excessively high amount, ductility is deteriorated. Therefore, the amount of Cr is to be limited to 2% or lower (more preferably 1.5% or lower), and the amount of Mo is to be limited to 1% or lower (more preferably 0.7% or lower).

[B: 0.0001 to 0.005%]

B is an element effective in improving hardening characteristics, and increasing the strength of the steel sheet when added in a minute amount. To exhibit such an effect, it is preferably contained in an amount of 0.0001% or higher. However, when the amount of B contained is excessively high and is higher than 0.005%, crystal grain boundaries may be embrittled and cracks may occur during rolling.

[Ca and/or REM: 0.003% or Lower (not Including 0%) in Total]

Ca and REM (rare earth element) are elements effective in controlling the form of sulfide in the steel and improving processability. The higher their amount contained, the higher the effects. However, when they are contained in an excessively high amount, the above-mentioned effect is saturated. Therefore, the amount is to be 0.003% or lower.

The method for producing the high strength steel sheet having a structure as mentioned above will be described now. In order to produce a high strength steel sheet as mentioned above, it is necessary to conduct a predetermined heat treatment by using a steel sheet in which the space factor of martensite and/or bainite (hereinafter these two phases may be referred to as “low-temperature transformation phases”) in total to the entire structure is 90% or higher and the former the grain size of austenite is 20 μm or smaller in terms of the equivalent of a circle diameter.

The material steel sheet used in the present invention has a space factor of the low-temperature transformation phase of 90% or higher. This low-temperature transformation phase may be constituted only by martensite or bainite. When the space factor of the low-temperature transformation phase is lower than 90%, and the material steel sheet is heated to a 2-phase range of the ferrite phase and austenite phase (dual-phase range annealing) in the annealing step (final annealing step) described later, a coarse ferrite phase and an austenite phase are produced. Therefore, the fine ferrite phase and martensite mentioned above cannot be obtained in the final structure. As a result, stretch-flanging performance cannot be improved.

A material steel sheet having space factor of the low-temperature transformation phase of 90% or higher can be produced by the following steps: First, a steel slab adjusted to meet the composition of chemical constituents as mentioned above is hot-rolled in such a manner that the finishing rolling temperature is higher than Ac₃ point. Second, the material steel sheet is cooled to a temperature lower than a martensite transformation start temperature, Ms point (temperature at which the austenite phase starts to transform into martensite), at an average cooling rate of 10° C./sec. or higher, and is then wound up, giving a material steel sheet having a space factor of martensite of 90% or higher. Moreover, a material steel sheet which is mainly composed of bainite and has a space factor of the low-temperature transformation phase of 90% or higher is obtained by cooling the material steel sheet to a bainite transformation temperature after the hot rolling, at an average cooling rate of 10° C./sec. or higher and winding up the same. When the finishing rolling temperature is Ac₃ point or lower or the cooling rate after the hot rolling is 10° C./sec. or lower, a ferrite phase is likely to be produced during cooling after the hot rolling, and therefore the space factor of the low-temperature transformation phase after the hot rolling is not 90% or higher.

In the above hot rolling step, from the perspective of micronization of the structure, it is preferable to appropriately adjust a predetermined heating temperature, and the time for retaining at the heating temperature (retaining time). In the present invention, the grain size of austenite is micronized by utilizing the pinning effect by finely depositing a micro-alloy (Ti, Nb, V, Zr, etc.). In order to do so, it is necessary to re-solutionize the deposition of the coarse micro-alloy produced prior to the hot rolling step. Accordingly, the heating temperature and its retaining time is preferably 1000° C. or higher, and 600 seconds or longer, respectively, to perform the solutionization effect of the micro-alloy (Ti, Nb, V, Zr and the like). When the heating temperature is 1400° C. or higher and its retaining time is longer than 1000 seconds, the grain size of austenite becomes undesirably coarse.

The material steel sheet used in the present invention needs to have the grain size of the former austenite of 20 μm or smaller. This is from the perspective of improvement of elongation and stretch-flanging performance due to the micronization of the structure. That is, by subjecting a basis steel sheet having a grain size of the former austenite of 20 μm or smaller to the final annealing step and tempering step, the final structure becomes finer than in the case where the grain size is larger than 20 μm, and elongation and stretch-flanging performance are significantly improved.

Even a steel sheet produced from a steel slab adjusted so as to meet the chemical components as mentioned above under conditions which do not meet the hot rolling and cooling rate conditions mentioned above can be imparted a space factor of the low-temperature transformation phase of 90% or higher by subjecting to the following preliminary annealing (Experiments No. 5, 6 in Table 14 described later).

This preliminary annealing is a treatment in which the above steel sheet is retained in a temperature range of Ac₃ point or higher for 5 seconds or longer, and is then cooled at an average cooling rate of 10° C./sec. or higher to a temperature of Ms point or lower or to the bainite transformation temperature range and retained. When the retaining temperature of the steel sheet is lower than Ac₃ point, a ferrite phase is likely to be produced, and a space factor of the low-temperature transformation phase of 90% or higher is not attained. Moreover, even in the case where the steel sheet is retained in a temperature range of Ac₃ point or higher, if the retaining time is shorter than 5 seconds, transformation of the metal structure into austenite is insufficient, and a space factor of 90% or higher is not attained.

By subjecting the material steel sheet whose structure and grain size of the former austenite have been adjusted in the manner described above to a heat treatment (final annealing step and tempering step) as described below, a high strength steel sheet whose space factors and grain sizes of the ferrite phase and martensite are appropriately adjusted can be obtained. At this time, the case where not only the preliminary annealing step but also acid cleaning, a cold rolling step and other processes are carried out between the hot rolling step and the heat treatment step described below also falls within the scope of the present invention. The actions and effects under the heat treatment conditions at this time are as follow:

First, a material steel sheet is subjected to a heat treatment in which it is heated to and retained at a temperature range of (Ac₃ point −100° C.) or higher but not higher than Ac₃ point for 1 second or longer but not longer than 2400 seconds, and is then cooled at a cooling rate of 10° C./sec. or higher to Ms point or lower (cooling stop temperature). By performing such an annealing step, a steel sheet having the structure (the space factor of ferrite: 5 to 30%, the space factor of martensite: 50 to 95%) mentioned above is obtained. Moreover, the mean crystal grain diameters of the ferrite phase and martensite in the high strength steel sheet which is finally obtained are determined by the sizes of the crystal grains of the ferrite phase and austenite produced when the material steel sheet is heated to and retained at a temperature range of (Ac₃ point −100° C.) or higher but not higher than Ac₃ point. That is, in order to obtain a fine dual phase steel sheet in which the mean grain size of the ferrite phase is 3 μm or smaller and the mean grain size of martensite is 6 μm or smaller, it is necessary to heat the material steel sheet to a temperature range of (Ac₃ point −100° C.) or higher but not higher than Ac₃ point and retain at the same.

In this annealing step, when the material steel sheet is heated and retained at a temperature range higher than Ac₃ point in which the austenite single phase is stable, crystal grains of austenite grow and combine with each other to become coarse, and the pinning effect of fine ferrite is not produced, whereby a fine dual phase steel sheet cannot be obtained. As a result, the stretch-flanging performance of the high strength steel sheet is lowered.

The “pinning effect” mentioned above is as follows: the basis steel sheet has a structure form mainly composed of a highly micronized lath-shaped low-temperature transformation phase due to the micronization effect of the micro-alloy. When such a steel sheet is heated to the high temperature side of the dual-phase range, a finely dispersed ferrite phase having a low space factor is produced. The term “ferrite phase” used in this invention denotes annealed martensite or annealed bainite produced when martensite or bainite is annealed at a high temperature (dual-phase range). Since such a ferrite phase suppresses the growth and combination of the austenite phase, the final structure obtained in the following hardening and tempering steps becomes a structure mainly composed of a very fine ferrite phase and martensite. When the material steel sheet is heated and retained at a temperature lower than (Ac₃ point −100° C.), transformation into austenite does not proceed sufficiently, and the space factor of martensite after the heat treatment becomes lower than 50%, thereby lowering the stretch-flanging performance of the steel sheet.

In this annealing step, if the heating and retaining time is shorter than 1 second, the austenite phase is not sufficiently produced, and therefore a space factor of martensite of 50% or higher cannot be attained after this annealing step. When the heating and retaining time is longer than 2400 seconds, the crystal grains of austenite produced become coarse, and therefore the fine dual structure mentioned above cannot be obtained. From such a perspective, the heating and retaining time in the final annealing needs to be in the range of 1 second or longer but not longer than 2400 seconds. It is preferably 5 seconds or longer but shorter than 1200 seconds.

When the cooling rate after heating and retaining is 10° C./sec. or lower or the cooling stop temperature is higher than Ms point, generation of bainite, retained austenite phase and pearlite, generation of more ferrite phase than necessary, and deposition of a cementite phase are caused, and structures other than martensite are formed in large amounts. Therefore, the space factor of martensite is lowered, and the space factor and mean crystal grain size of the ferrite phase are excessively increased, leading to lowered elongation and stretch-flanging performance. The higher the cooling rate at this time, and the lower the cooling stop temperature, the higher the space factor of martensite is likely to be. However, since the temperature and time of the above dual-phase range annealing are suitably controlled, the space factor becomes no greater than 95%.

After the annealing step as mentioned above is performed, it is necessary to carry out tempering (reheating treatment) in which the material steel sheet is retained at a temperature range of 300 to 550° C. for 60 seconds or longer but not longer than 1200 seconds. In the steel sheet which has undergone the annealing step as mentioned above, fine (ferrite phase+martensite) is formed in its metal structure. However, martensite in an annealed state is very hard, which lowers elongation. Moreover, since martensite is hard, a difference in hardness between martensite and soft ferrite is large, which leads to lowered stretch-flanging performance. In order to obtain excellent elongation and stretch-flanging performance, the hardness of martensite needs to be reduced than in an annealed state, which is why it is subjected to the tempering step.

When the retaining temperature in this tempering step is lower than 300° C., softening of martensite is insufficient, and therefore elongation and stretch-flanging performance of the steel sheet are lowered. In contrast, when the retaining temperature is higher than 550° C., a coarse cementite phase is deposited, whereby the stretch-flanging performance of the steel sheet is lowered.

When the retaining time of the tempering step is shorter than 60 seconds, softening of martensite is insufficient, and therefore elongation and stretch-flanging performance of the steel sheet are lowered. In contrast, when the retaining time is longer than 1200 seconds, martensite is too softened so that ensuring strength is made difficult, and the stretch-flanging performance of the steel sheet is lowered by the deposition of cementite. This retaining time is preferably 90 seconds or longer but not longer than 900 seconds, and more preferably 120 seconds or longer but not longer than 600 seconds.

By subjecting the material steel sheet as mentioned above to annealing (final annealing) and tempering as mentioned above, a steel sheet in which the space factors and grain sizes of the ferrite phase and martensite are suitably adjusted can be obtained, and a tensile strength as high as 590 MPa and excellent elongation and stretch-flanging performance are achieved. Such a high strength steel sheet can be used as a steel sheet with excellent press formability as a material for various steel products typically including automobiles.

Example 3

Now, the present invention will be described more specifically by referring to Examples. The present invention is not restricted in itself by the following Examples. Therefore, it is possible to carry out the invention by properly modifying the Examples within the above described or later describe spirit of the invention, and such modifications are all to be included in the technical scope of the present invention.

steel slabs having compositions of chemical constituents shown in Tables 10 and 11 below were prepared, and material steel sheets were prepared from the steel slabs under the hot rolling conditions and preliminary annealing conditions shown in Tables 12 and 13 below. Tables 10 and 11 also show the point (Ac₃ transformation point) and martensite transformation start temperature, Ms point, for each steel type determined by equations (1) and (2).

AC ₃(° C.)=910−203·√[C]−15.2·[Ni]+44.7·[Si]+104·[V]+31.5·[Mo]+13.1·[W]−330·[Mn]+11·[Cr]+20·[Cu]−720·[P]−400[Al]−120·[As]−400·[Ti]  (1)

Ms(° C.)=550−361·[C]−39·[Mn]−35·[V]−20·[Cr]−17·[Ni]−10·[Cu]−5·[Mo]−5[W]+15·[Co]+30·[Al]  (2)

However, [C], [Ni], [Si], [V], [Mo], [W], [Mn], [Cr], [Cu], [P], [Al], [As], [Ti] and [Co] represent the amounts contained of C, Ni, Si, V, Mo, W, Mn, Cr, Cu, P, Al, As, Ti and Co (% by mass), respectively.

TABLE 10 Steel Composition of chemical components* (% by mass) Transformation point type C Si Mn P S Al Ti Nb V Zr Others Ac₃ (° C.) Ms (° C.) A 0.12 1.23 1.58 0.010 0.001 0.045 — — — — — 872 446 B 0.25 1.57 2.24 0.013 0.002 0.041 — — — — — 837 374 C 0.01 0.87 1.53 0.012 0.002 0.042 — — 0.015 — — 909 487 D 0.08 1.87 2.21 0.016 0.001 0.037 — — 0.020 — — 898 435 E 0.25 1.51 2.05 0.020 0.002 0.031 — — 0.016 — — 843 380 F 0.35 1.49 1.98 0.012 0.002 0.032 — — 0.012 — — 820 347 G 0.18 0.05 2.03 0.009 0.001 0.029 0.022 — — — — 797 407 H 0.16 2.63 1.20 0.009 0.002 0.033 0.037 — — — — 945 446 I 0.21 3.54 2.08 0.011 0.001 0.038 — 0.011 — — — 936 394 J 0.13 1.51 0.38 0.009 0.003 0.039 — 0.015 — — — 915 489 K 0.12 1.49 0.62 0.009 0.001 0.031 — 0.021 — — — 906 483 L 0.22 1.21 2.78 0.006 0.002 0.031 — — — 0.021 — 802 363 M 0.20 1.50 3.49 0.013 0.001 0.033 — — — 0.022 — 804 343 N 0.17 1.35 2.02 0.015 0.002 0.005 — — 0.018 — — 840 409 O 0.19 1.32 1.99 0.011 0.003 0.087 — — 0.017 — — 865 406 P 0.18 1.43 2.04 0.012 0.001 0.162 — — 0.012 — — 901 410 Q 0.17 1.32 1.92 0.012 0.002 0.040 0.005 — — — — 854 415 R 0.18 1.28 2.12 0.014 0.002 0.038 — 0.001 0.001 — — 843 403 *Remainder: Iron and inevitable impurities other than P and S

TABLE 11 Steel Composition of chemical components* (% by mass) Transformation point type C Si Mn P S Al Ti Nb V Zr Others Ac₃ (° C.) Ms (° C.) A1 0.17 1.40 2.05 0.009 0.002 0.028 0.015 — — — — 851 410 B1 0.17 1.20 2.20 0.010 0.001 0.031 — 0.018 — — — 833 404 C1 0.19 1.45 2.10 0.009 0.001 0.031 — — 0.021 — — 844 400 D1 0.20 1.37 2.04 0.015 0.003 0.027 — — — 0.015 — 841 399 E1 0.17 1.35 2.00 0.011 0.002 0.035 0.016 0.012 — — — 855 412 F1 0.18 1.37 2.13 0.010 0.002 0.037 0.011 — 0.022 — — 850 402 G1 0.19 1.34 2.06 0.010 0.002 0.031 0.005 0.015 0.004 — — 841 402 H1 0.16 1.26 1.78 0.014 0.002 0.035 — — 0.150 — — 872 419 I1 0.17 1.34 1.99 0.009 0.002 0.028 — — 0.014 — Ni: 0.2 845 411 J1 0.16 1.41 1.90 0.010 0.001 0.031 — — 0.023 — Cu: 0.1 857 418 K1 0.17 1.32 2.18 0.012 0.002 0.040 — — 0.025 — Cr: 0.35 843 397 L1 0.16 1.26 2.00 0.015 0.003 0.042 — — 0.019 0.03 Mo: 0.1 858 414 M1 0.19 1.32 1.95 0.016 0.003 0.045 — — 0.025 — B: 0.0002 854 406 N1 0.16 1.39 1.95 0.013 0.002 0.039 — — 0.017 — Ca + REM: 0.001 859 417 O1 0.19 1.33 1.89 0.009 0.003 0.039 — 1.135 — — — 846 409 P1 0.21 1.35 2.08 0.011 0.003 0.037 0.154 — 0.404 0.551 — 941 380 *Remainder: Iron and inevitable impurities other than P and S

TABLE 12 Hot-rolling conditions Preliminary annealing conditions Heating Retaining Hot finishing Winding Heating Retaining Cooling Cooling stop Experiment Steel temperature time temperature temperature temperature time rate temperature No. type (° C.) (sec.) (° C.) (° C.) (° C.) (sec.) (° C./sec.) (° C.) 1 A 1300 1800 930 550 850 120 500 20 2 B 1250 1800 950 550 880 240 300 20 3 C 1300 2400 850 600 900 90 300 20 4 D 1300 1800 850 550 930 240 200 20 5 E 1200 1800 880 250 — — — — 6 F 1200 1200 850 300 — — — — 7 G 1300 2400 800 500 900 60 200 20 8 H 1250 1800 850 600 930 120 300 20 9 I 1300 1800 900 550 930 120 100 50 10 J 1300 1800 900 500 910 60 100 20 11 K 1200 2400 900 550 930 120 500 20 12 L 1300 1800 850 550 930 360 300 20 13 M 1200 1800 850 500 850 120 100 20 14 N 1300 1800 880 500 860 120 300 20 15 O 1300 2400 900 500 880 10 300 20 16 P 1300 1800 900 550 930 180 100 50 17 Q 1250 1800 850 550 880 120 300 20 18 R 1250 1800 850 550 930 180 300 20

TABLE 13 Hot-rolling conditions Preliminary annealing conditions Heating Retaining Hot finishing Winding Heating Retaining Cooling Cooling stop Experiment Steel temperature time temperature temperature temperature time rate temperature No. type (° C.) (sec.) (° C.) (° C.) (° C.) (sec.) (° C./sec.) (° C.) 19 A1 1300 1800 850 550 880 120 300 20 20 B1 1200 1800 850 550 850 60 300 20 21 C1 1300 1800 850 550 850 0 300 20 22 D1 1300 1800 850 550 880 120 300 20 23 E1 1300 1800 900 550 870 30 300 20 24 F1 1300 1800 900 550 860 90 300 20 25 G1 1300 1800 900 550 850 60 300 20 26 H1 1350 1800 900 550 890 120 300 20 27 I1 1250 1800 850 550 860 120 300 20 28 J1 1300 1800 870 550 875 120 300 20 29 K1 1200 1800 850 550 870 10 300 20 30 L1 1300 2400 870 550 860 120 300 20 31 M1 1300 1200 850 550 900 120 300 20 32 N1 1300 1200 850 550 870 120 300 20 33 O1 1200 1800 850 550 930 240 200 20 34 P1 1300 1200 950 550 950 120 200 20 35 B1 1300 1800 850 550 900 120 300 20 36 C1 1300 1800 850 550 880 120 300 20

The material steel sheets obtained were subjected to the final annealing and reheating (tempering) under the conditions shown in Tables 14 and 15 below to prepare test steel sheets, and the structures (space factor of ferrite α, mean grain size of ferrite α, space factor of martensite M, and mean grain size of martensite M) and mechanical characteristics (tensile strength TS, elongation EL, hole expansion rate λ) of the test steel sheets were determined by the methods described below. Tables 14 and 15 below also show the structures [phase constitution, space factor of low-temperature transformation phase, grain size of former austenite (γ)] of the test steel sheets before the final annealing.

[Method for Measuring Structures of Test Steel Sheets]

The space factors of ferrite α and martensite M were determined by subjecting the structure pictures of the test steel sheets after being corroded with nital to image analysis. The mean grain sizes of ferrite α and martensite M were measured by structure analysis using FE/SEM-EBSP, and the measurements were converted into “the equivalent of a circle diameter” described above to determine their mean value.

[Method for Measuring Mechanical Characteristics of Test Steel Sheets]

(a) Tensile test: A universal tensile tester manufactured by Instron was used to determine tensile strength (TS) and elongation (total elongation rate: EL) by using JIS No. 5 tensile test pieces.

(b) Hole expansion test: 20-ton hole expansion tester manufactured by Tokyo Koki was used to determine hole expansion rates (λ) according to Japan Iron and Steel Federation standard (JFST1001-1996), and stretch-flanging performance was evaluated.

TABLE 14 Structure before final annealing Space factor of Grain size Final annealing conditions Tempering conditions low-temperature of Heating Retaining Cooling Cooling stop Heating Retaining Experiment Steel Phase transformation phase former γ temperature time rate temperature temperature time No. type constitution* (% by volume) (μm) (° C.) (sec.) (° C./sec.) (° C.) (° C.) (sec.) 1 A M 100 28 850 120 500 20 400 180 2 B M 100 29 810 120 300 20 400 120 3 C M 100 18 850 240 100 20 500 180 4 D M 100 11 870 120 200 20 500 180 5 E M 93 13 815 90 300 20 520 120 6 F M 97 17 810 240 100 20 350 180 7 G M 100 12 750 120 200 100 400 120 8 H α + M 95 11 910 360 300 50 500 180 9 I α + M 95 16 870 120 100 20 350 120 10 J α + M 95 14 900 90 100 20 450 180 11 K M 100 12 850 180 500 20 520 180 12 L M 100 12 770 120 300 20 500 180 13 M M 100 13 795 120 100 20 400 180 14 N M 100 10 820 120 300 20 500 180 15 O M 100 14 850 120 300 20 500 180 16 P M 100 9 880 120 100 50 400 120 17 Q M 100 24 830 120 300 20 500 180 18 R M 100 29 830 120 300 20 400 120 *M: Martensite, α: Ferrite, γ: Austenite

TABLE 15 Structure before final annealing Space factor of Grain size Final annealing conditions Tempering conditions low-temperature of Heating Retaining Cooling Cooling stop Heating Retaining Experiment Steel Phase transformation phase former γ temperature time rate temperature temperature time No. type constitution* (% by volume) (μm) (° C.) (sec.) (° C./sec.) (° C.) (° C.) (sec.) 19 A1 M 100 12 830 180 300 20 500 180 20 B1 M 100 9 825 120 300 20 500 240 21 C1 M 100 7 800 120 300 20 500 180 22 D1 M 100 12 810 120 300 20 500 180 23 E1 M 100 10 790 180 300 20 520 180 24 F1 M 100 11 810 180 300 20 500 180 25 G1 M 100 9 810 180 300 20 500 180 26 H1 M 100 10 840 240 300 20 500 120 27 I1 M 100 12 825 120 300 20 500 120 28 J1 M 100 11 830 120 300 20 500 180 29 K1 M 100 8 810 120 300 20 500 180 30 L1 M 100 9 850 60 300 20 500 180 31 M1 M 100 13 820 120 300 20 500 180 32 N1 M 100 10 830 120 300 20 500 180 33 O1 M 100 16 830 180 200 50 500 180 34 P1 M 100 14 900 120 200 50 500 180 35 B1 M 100 15 730 120 300 20 500 120 36 C1 M 100 13 860 120 300 20 400 120 *M: Martensite, α: Ferrite, γ: Austenite

The measurement results of the structures (space factor of ferrite α, mean grain size of ferrite α, space factor of martensite M, mean grain size of M), and mechanical characteristics (tensile strength TS, elongation EL, hole expansion rate λ) of the test steel sheets are shown in Tables 16 and 17 below. As for “evaluation” of the mechanical characteristics, tensile strength (TS) of 590 MPa or higher, elongation (EL) of 10% or higher, and hole expansion rate (λ) of 80% or higher were rated excellent characteristics. The samples which were excellent in all three characteristics were rated o; those which were excellent in two characteristics out of three were rated Δ; and those which were excellent in only one characteristic out of three were rated x. Only ∘ was rated a pass.

TABLE 16 Structure of steel plate Mechanical characteristics Space factor Mean grain Space factor Mean grain Tensile Experiment Steel of α size of α of M size of M strength TS Elongation EL λ No. type (% by volume) (μm) (% by volume) (μm) (MPa) (%) (%) Evaluation 1 A 12 4.8 88 4.3 609 28.5 67.3 Δ 2 B 13 4.9 87 4.5 1341 7.9 71.4 X 3 C 19 2.9 81 2.5 548 32.1 82.5 Δ 4 D 12 2.1 88 2.2 993 12.1 113.0 ◯ 5 E 13 2.3 85 2.0 1107 11.9 108.0 ◯ 6 F 11 2.7 89 2.1 1398 6.1 54.2 X 7 G 16 2.8 84 2.4 776 17.9 107.3 ◯ 8 H 14 2.3 86 2.3 1024 13.2 105.0 ◯ 9 I 28 3.0 72 2.3 1419 5.8 27.4 X 10 J 35 3.8 65 2.8 587 29.0 65.9 X 11 K 25 2.8 75 2.4 603 28.3 86.3 ◯ 12 L 14 2.5 86 2.1 1117 12.1 100.3 ◯ 13 M 12 2.5 88 2.2 1311 7.9 59.9 X 14 N 11 1.6 89 1.7 1024 14.2 125.0 ◯ 15 O 10 1.3 90 1.5 1018 15.4 128.3 ◯ 16 P 13 2.5 87 2.0 1032 10.1 67.3 Δ 17 Q 12 4.0 88 3.9 1098 12.1 72.9 Δ 18 R 11 4.7 89 4.1 1015 10.8 69.5 Δ * M: Martensite, α: Ferrite

TABLE 17 Structure of steel plate Mechanical characteristics Space factor Mean grain Space factor Mean grain Tensile Experiment Steel of α size of α of M size of M strength TS Elongation EL λ No. type (% by volume) (μm) (% by volume) (μm) (MPa) (%) (%) Evaluation 19 A1 12 2.0 88 1.9 1022 11.9 112.0 ◯ 20 B1 9 1.6 91 1.5 995 13.2 117.2 ◯ 21 C1 26 1.8 74 1.9 978 14.2 119.9 ◯ 22 D1 12 2.1 88 2.0 1017 12.5 112.7 ◯ 23 E1 22 2.2 78 1.9 716 16.2 97.3 ◯ 24 F1 16 2.0 84 2.1 1012 14.9 118.6 ◯ 25 G1 13 1.7 87 1.9 1023 13.6 124.1 ◯ 26 H1 8 1.4 92 1.3 1100 13.8 118.8 ◯ 27 I1 10 1.7 90 1.9 1025 14.3 123.6 ◯ 28 J1 11 1.8 89 1.8 1098 13.7 121.8 ◯ 29 K1 12 1.9 88 1.9 1167 11.6 112.1 ◯ 30 L1 7 1.1 93 1.3 1228 10.9 98.5 ◯ 31 M1 12 2.0 88 1.9 1145 11.0 103.5 ◯ 32 N1 12 1.8 88 1.8 1097 11.7 105.2 ◯ 33 O1 13 2.7 87 2.1 1212 8.4 22.9 X 34 P1 16 2.2 84 1.9 1329 5.3 19.1 X 35 B1 78 13.2 22 3.5 578 15.3 34.4 X 36 C1 0 — 100 3.8 1383 5.9 78.5 X * M: Martensite, α: Ferrite

It is possible to consider as follows from these results: Since the samples of Experiments No. 4, 5, 7, 8, 11, 12, 14, 15 and 19 to 32 all meet the requirements defined in the present invention, they are all provided with excellent characteristics.

In contrast, the test pieces of No. 1 to 3, 6, 9, 10, 13, 16 to 18 and 33 to 36 are not provided with satisfactory characteristics as the followings because at least one requirement of their composition of chemical constituents and manufacturing conditions falls outside the scope defined in the present invention.

Since the test pieces of Experiments No. 1, 2 do not contain Ti, Nb, V, Zr and the like, the grain size of the former γ in the material steel sheet (steel sheet before the final annealing) became coarse, and the desired elongation and stretch-flanging performance could not be obtained.

The test piece of Experiment No. 3 has low tensile strength TS since the amount of C contained does not fall with in the preferable range defined in the present invention. The test piece of Experiment No. 6 has strength higher than necessary because the amount of C contained is higher than the preferable range defined in the present invention, so that ductility is lowered and elongation characteristics are deteriorated.

In the test piece of Experiment No. 9, the amount of Si contained is higher than the preferable range defined in the present invention, and therefore its ductility is lowered, and elongation and stretch-flanging performance are deteriorated.

In the test piece of Experiment No. 10, the amount of Mn contained does not fall with in the preferable range defined in the present invention, and therefore the space factor of ferrite is increased, deteriorating tensile strength and stretch-flanging performance.

In the test piece of Experiment No. 13, the amount of Mn contained is higher than the preferable range defined in the present invention, and therefore its ductility is lowered, deteriorating elongation and stretch-flanging performance.

In the test piece of No. 16, flaws on the surface of the steel material are increased because the amount of Al is higher than the preferable range defined in the present invention, whereby ductility of flawed material is lowered, and stretch-flanging performance is deteriorated.

In the test pieces of Experiments Nos. 17 and 18, the amounts of Ti, Nb, V, Zr and the like contained are low. Therefore, micronization has not been sufficiently produced, and desired stretch-flanging performance has not been obtained.

In the test pieces of Experiments Nos. 33 and 34, the amounts of Ti, Nb, V, Zr and the like contained are too high. Therefore, coarse carbide remains even under predetermined heat treatment conditions, and elongation and stretch-flanging performance are deteriorated.

In the test piece of Experiment No. 35, the heating temperature in the final annealing is much below the range defined in the present invention. Therefore, the space factor and mean grain size of ferrite, the space factor and mean grain size of martensite in the final structure fall outside the range defined in the present invention, and desired tensile strength and stretch-flanging performance have not been obtained.

The test piece of Experiment No. 36, the heating temperature in the final annealing is much above the range defined in the present invention. Therefore, the final structure became a single-phase structure of martensite, and the space factor of ferrite and the space factor and mean grain size of martensite fall outside the range defined in the present invention. Accordingly, desired elongation and stretch-flanging performance have not been obtained.

Although the present invention has been described in detail with reference to specific embodiments, it is obvious for a person of skill in the art that various modifications and alterations can be made without deviating from the spirit and scope of the present invention. the present application is based on Japanese Patent Application (No. 2006-194056) applied on Jul. 14, 2006; Japanese Patent Application (No. 2007-144466) applied on May 31, 2007; Japanese Patent Application No. applied on May 31, 20072007-144705); and Japanese Patent Application (No. 2007-145987) applied on May 31, 2007, and their disclosures are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The high strength steel sheet according to the present invention has excellent elongation and stretch-flanging performance at the same time, and thus has excellent press formability. Therefore, the high strength steel sheet according to the present invention can be processed by press molding to be used for various industrial products such as automobiles, especially for industrial products where weight reduction is necessary. 

1. A high strength steel sheet which comprises, in percent by mass, C: 0.05 to 0.3%; Si: 3% or less (not including 0%.); Mn: 0.5 to 3.0%; Al: 0.01 to 0.1%; and the remainder comprising iron and inevitable impurities, the high strength steel sheet having a space factor of a martensite phase which is a main component of a metal structure of 50% or higher, and a tensile strength of 590 MPa or higher.
 2. The high strength steel sheet according to claim 1, wherein the structure which is a main part of the metal structure is the martensite phase, which is tempered martensite, and finely dispersed annealed bainite; the space factor of the tempered martensite is 50 to 95%; the space factor of the annealed bainite is 5 to 30%; and a mean grain size of the tempered martensite is 10 μm or lower in terms of the equivalent of a circle diameter.
 3. The high strength steel sheet according to claim 1, wherein the space factor of the martensite phase which is a main part of the metal structure is 80% or higher; the mean grain size of the martensite phase is 10 μm or smaller in terms of the equivalent of a circle diameter; the space factor of the martensite phase having a grain size of 10 μm or larger in terms of the equivalent of a circle diameter in the martensite phase is 15% or lower; and the space factor of the retained austenite phase in the metal structure is 3% or lower.
 4. The high strength steel sheet according to claim 3, wherein the martensite phase is a tempered martensite phase; an annealed martensite phase is contained as the metal structure other than the martensite phase and the retained austenite phase; and the space factor of the annealed martensite phase is 3 to 20%.
 5. The high strength steel sheet according to claim 1, wherein the structure which is a main part of the metal structure is the martensite phase and a ferrite phase; the space factor of the martensite phase is 50 to 95%; the space factor of the ferrite phase is 5 to 30%; and the mean grain size of the martensite phase is 10 μm or smaller in terms of the equivalent of a circle diameter.
 6. The high strength steel sheet according to claim 5, wherein the ferrite phase is annealed martensite.
 7. The high strength steel sheet according to claim 1, wherein at least an element selected from Ti, Nb, V and Zr is further contained in an amount of 0.01 to 1% by mass in total.
 8. The high strength steel sheet according to claim 1, wherein Ni and/or Cu are/is further contained in an amount of 1% by mass or lower in total.
 9. The high strength steel sheet according to claim 1, wherein Cr: 2% by mass or less and/or Mo: 1% by mass or less are/is further contained.
 10. The high strength steel sheet according to claim 1, wherein 0.0001 to 0.005% by mass of B is further contained.
 11. The high strength steel sheet according to claim 1, wherein Ca and/or REM are/is further contained in an amount of 0.003% by mass or lower in total.
 12. A method for manufacturing a high strength steel sheet according to claim 2, the method comprising using a steel sheet having a space factor of bainite in the entire metal structure of 90% or higher as a material steel sheet; heating and retaining the material steel sheet at a temperature of (Ac₃ point −100° C.) or higher but not higher than Ac₃ point for 0 to 2400 seconds (including 0 seconds); then cooling the material steel sheet to a transformation start temperature of martensite, Ms point, or lower at an average cooling rate of 10° C./sec. or higher; and subsequently conducting a heat treatment in which the steel sheet is heated and retained at a temperature of 300 to 550° C. for 60 to 1200 seconds.
 13. A method for manufacturing a high strength steel sheet according to claim 3, the method comprising using a steel sheet in which the total space factor of the martensite phase and/or of the retained austenite phase in the entire metal structure is 90% or higher as a material steel sheet; heating and retaining the steel sheet at a temperature of (Ac₃ point −100° C.) or higher but not higher than Ac₃ point for 30 to 1200 seconds; cooling the steel sheet to a transformation start temperature of martensite, Ms point, or lower at an average cooling rate of 10° C./sec. or higher; and further conducting a heat treatment in which the steel sheet is heated and retained at a temperature of 300 to 500° C. for 60 to 1200 seconds.
 14. A method for manufacturing a high strength steel sheet according to claim 5, the method comprising providing a total space factor of the martensite phase and/or bainite phase in the entire metal structure is 90% or higher; using a steel sheet having a grain size of the former austenite of 20 μm or smaller in terms of the equivalent of a circle diameter as a material steel sheet; heating and retaining the steel sheet at a temperature of (Ac₃ point −100° C.) or higher but not higher than Ac₃ point for 1 to 2400 seconds; then cooling the steel sheet to a transformation start temperature of martensite, Ms point, or lower at an average cooling rate of 10° C./sec. or higher; and subsequently conducting a heat treatment in which the steel sheet is heated and retained at a temperature of 300 to 550° C. for 60 to 1200 seconds. 